In memoriam Ivar Holand Preface 1 State of the art 1.1 Historical overview 1.2 Design concepts 1.3 Development of the concrete material 1.4 Design 1.5 Construction methods 1.6 Rules and regulations 1.7 Quality assurance 1.8 Durability 1.9 Competitiveness 1.10 Removal. Demolition. Recycling 1.11 Spin-off effects References 2 Concept definition and project organization 2.1 Objectives 2.2 General description of an offshore concrete structure 2.3 Project phases 2.4 Rules and regulations 2.5 Project management 2.6 Work during early phases of a project 2.7 The concept definition phase 2.8 Project organization phase References 3 Simplified analyses 3.1 Introduction 3.2 Analysis activities 3.3 Classification of load effects 3.4 Gravity base structures 3.5 Floating structures 3.6 Ship impact 3.7 Non-linear geometric effects References
4 Global analyses 4.1 Objective 4.2 Linear finite element methods 4.3 From linear analysis to design 4.4 Postprocessing 4.5 Non-linear analyses 4.6 Verification References 5 Design 5.1 Typical structures and structural parts 5.2 Design documents 5.3 Design procedures 5.4 Reinforcement References 6 Quality assurance 6.1 Introduction 6.2 Basic requirements for quality assurance 6.3 Quality assurance in engineering and design of concrete structures 7 Verification of design 7.1 Introduction 7.2 Norwegian Petroleum Directorate requirements 7.3 Levels of verification 7.4 External verification 7.5 Internal verification 7.6 Budgeting, reporting and follow-up of non-conformances 7.7 Requirements concerning qualifications 7.8 Scope of verification activity References Appendices Appendix A Appendix B Appendix C Appendix D Appendix E Appendix F Appendix G Appendix H Discipline Activity Model Discipline Check (DC) Inter Discipline Check (IDC) Verification Design Review (DR) Hazard and Operability Analysis (HAZOP) Worst-Case Analysis Quality Audit/Quality System Audit
Ivar Holand was on several occasions appointed by the Norwegian government and by Statoil to investigate accidents related to the offshore industry. Ivar Holand was a member of The Royal Norwegian Society of Sciences and Letters (from 1969) and an honorary member of several associations. In 1982 he was appointed an honorary doctor of Chalmers Institute of Technology in Gothenburg, and in 1995 he was knighted with the order of St. Olav, the highest honour in Norway. Professor Holand was the teacher and inspiration of a whole generation of structural engineers in Norway. Those of us who met him and worked with him will remember him with the greatest respect and gratitude for his professional achievements as well as his outstanding personal qualities.
Preface
Construction of offshore concrete platforms has to a large extent been a North Sea speciality with the majority of the activity being undertaken in Norway and UK. When this activity started around 1970, there were no standards available, specifically written for offshore concrete structures. This fact motivated development work in this field. Moreover, there were large potential economic gains from improving the concrete material and the design and production methods. The development of high strength/high performance concrete and improvement of analysis and design methods were very much stimulated by this situation. The situation also required the development of regulations (on a government level), standards and company specifications to certify that the safety and reliability of the structures was adequate. Norwegian standards were therefore revised with the particular objective of becoming suitable for offshore concrete structures. The activities in Norway have provided the general background for the present book. Thus, Norwegian regulations, standards and specifications have frequently been referred to in the book. When the book is used in other countries, the rules and regulations in the country in question must be followed. In such cases, references to Norwegian rules and standards may be used as illustrative examples of how various issues may be handled. Other countries that have had similar interests and activities (but hardly on the same scale as Norway) are the UK and Canada. Furthermore, there has been interest in offshore concrete structures in France (design), the Netherlands and Australia, and rules and standards from these countries are referred to when relevant. ISO standards in this field are in drafting stage. Other relevant international documents are so far available only to a limited extent. For the topic Quality Assurance (Chapter 6 and parts of Chapter 7) the situation is completely different. ISO standards in this field are in a mature stage and have been referred to. Chapter 6 explains how the well-known QA methodology is applied to offshore concrete structures. The present book is based on the technical contents in “Manual for design of offshore concrete structures” prepared by the Norwegian oil company Statoil, Stavanger, Norway, in cooperation with The Foundation for Scientific and Industrial Research (SINTEF) at the University of Trondheim, Norway. A first edition of this manual was issued in Norwegian in 1993. Urged by and with the economic support from the Japan Society of Civil Engineers, the manual was translated into English, and an English version was issued by Statoil in June 1996.The editors wish to thank Statoil for the permission to use this English version of the manual as the basis for the present book. Although the manual has been used as a basis, the content has been completely reworked. Previous chapters and appendices have been combined, parts of the previous appendices have been omitted and a new introductory chapter added. Emphasis is given to presenting an overview of important problem areas in design, and to present specific recommendations to ensure the fulfilment of a satisfactory product. However, due among other things to restrictions on the extent of the volume of the book, the following topics are only briefly dealt with:
• Concrete platform removal, see Section 1.10, which necessitate that removal procedures are designed into the structure from the start • Durability of concrete structures in sea water, see Section 1.8, where good experience has been acquired in the North Sea • Construction procedures, see Section 1.5 • Geotechnical stability, which is essential for the operation of a safe and reliable structure • Deficiencies related to corrosion of piping systems, and the need for repair procedures for this types of piping. Different authors have been responsible for the various chapters of this book as indicated in the chapter headings. The contributors are: Professor, Dr. Ove T.Gudmestad, Statoil, Stavanger, Norway: Chapter 2 Professor, Dr. Ivar Holand, SINTEF: Chapters 1 and 4 Senior Scientist, M.Sc. Erik Jersin, SINTEF, Trondheim, Norway: Chapter 6 Professor, Dr. Tore H.Søreide, Reinertsen Engineering and the Norwegian University of Science and Technology, Trondheim, Norway: Chapters 3 and 7 Professor Erik Thorenfeldt, SINTEF: Chapter 5 The editors thank their co-authors and the publisher for their excellent co-operation during the entire process of preparing the manuscript.
Trondheim/Stavanger, Spring 2000
Ivar Holand
Ove T.Gudmestad
Erik Jersin
Note: After the sad loss of Ivar Holand, his son Per Holand volunteered to read the final proofs of the manuscript. Without his enthusiastic effort in the final phase, the publishing of this book would have been considerably delayed. Many thanks, Per!
1 State of the art
Ivar Holand, SINTEF
1.1 Historical overview The beginning of the story of the remarkable offshore concrete structures is only 30 years behind us. When the petroleum industry established activities in the North Sea in the late sixties, an immediate reaction from the Norwegian construction industry was that concrete should be able to compete with steel, that had been the traditional structural material in this industry (Fjeld and Morley, 1983), (Moksnes, 1990), (Gudmestad, Warland, and Stead, 1993). This assumption proved to be true regarding the cost of the structure as well as the maintenance costs. One after the other of spectacular structures, 22 in total, have been placed on the sea bed in the North Sea reaching up to 30 m above sea level and down to 303 m at the deepest location, making this structure one of the tallest concrete structures in the world (Holand and Lenschow, 1996). (A general description of an offshore concrete structure is also found in Chapter 2.) The most innovative period was around 1970, when the Ekofisk concrete platform was towed to its location (Fig. 1.1), and the first of the many Condeep platforms was on the drawing board.
Offshore concrete structures have proved to represent a competitive alternative for substructures in the North Sea and in other places where large offshore structures for production of oil and/or gas are required. The deep Norwegian fjords have represented a particular advantage during the construction phase, as the substructures here can be lowered deep into the sea, enabling the production plant to be floated on barges over the platform for transfer to the substructure. Hence, the production plant can be completed at quay side where the productivity is best. Hereby, costly offshore heavy lifting and hook-up activities are avoided. Furthermore, offshore concrete structures have proved to be highly durable and to have good resistance against corrosion (Fjeld and Morely, 1983), provided that the concrete is dense, have a minimum of cracks and sufficient cover over the rebars. The Norwegian Standard NS 3473 requires 40 mm for permanently submerged parts and 60 mm in the splash zone. In the North Sea even larger rebar covers have normally been used. Recent concrete projects are: • in the Netherlands: F3, concrete gravity base 1992 • in the North Sea, Norwegian sector: Troll gas fixed platform (Fig. 1.2), Heidrun tension leg platform (Fig. 1.3) and Troll oil catenary anchored floating oil platform (Fig. 1.4), all completed in 1995 • in the North Sea, British sector: The BP Harding Gravity Base Tank completed in 1995 • in Congo: N’Kossa, concrete barge 1995 • in Australia: Wandoo B, Bream B, West Tuna, concrete substructures completed 1996 • on the Canadian continental shelf outside Newfoundland: Hibernia 1997 • in the North Sea, Danish sector: South Arne, to be completed in 1999. Although the recent development has not favoured concrete platforms, there are several concept studies ongoing in the design offices. As promising floater concepts, new generations of tension leg platforms and a concrete Spar shall be mentioned. (Chabot, 1997), (Brown and Nygaard, 1997). At present work is ongoing to develop more cost-efficient concrete structures for development of smaller hydrocarbon fields. The F3 field in the Dutch offshore sector, mentioned above, is an example; a concrete structure installed at Ravenspurne North in the British sector is another. 1.2 Design concepts
Mobil at the Hibernia field in Canadian waters and completed in 1997 is also mainly of the same type. 1.2.2 Condeeps and similar gravity based structures The next concept, the Condeep, which became the winning concept for a period of time, was based on a cellular base with circular cells and one to four hollow columns (shafts), and thus had the advantage of a slim shape through the wave zone. Beryl Alpha, the first Condeep platform, was placed on the UK continental shelf in 1975. Up to 1995 a total of 14 Condeeps have been installed in the North Sea (Ågnes, 1997). Fig. 1.2 shows the largest of these structures. Other designs were based on the same principles, except that the cells in the raft were rectangular (four platforms in the North Sea completed 1976–78, and also BP Harding in UK waters, 1995, and South Arne on Danish Continental shelf, 1999). 1.2.3 Tension leg floaters As the exploitation of hydrocarbons moved to deeper waters, structures carried by buoyancy became more competitive than gravity based structures. For the first concrete tension leg platform, the Heidrun platform (Fig. 1.3) installed in 1995 in 345 m of water, the complete hull, including the main beams carrying a steel deck, is made of high performance lightweight aggregate concrete. The structure received the FIP (Fédération Internationale de la Précontrainte) award for outstanding structures 1998 (FIP 1998). 1.2.4 Catenary anchored floaters
Depending on several factors (depth, wave conditions, etc.) a catenary anchoring may be preferred. The first concrete platform of this type is shown in Fig. 1.4. 1.2.5 New concepts
Future concrete structures will most probably be based on a variety of new concepts (Ågnes, 1997), (Olsen, 1999), e.g.: • • • • Jack-up foundations (ex. BP Harding in the UK sector of the North Sea (O’Flynn, 1997)) Anchorage Foundations for Tension Leg Platforms Spar buoys Lifting vessels for removal
Fig. 1.2 Troll Gas, the largest platform of the CONDEEP type (by courtesy of Aker Maritime) • • • • completed 1995 water depth 303 m height of concrete structure 369.4 m concrete volume 234 000 m3
Fig. 1.3 Heidrun, the first tension leg floater with a concrete hull (by courtesy of Aker Maritime) • • • • completed 1995 hull draft at field 77 m concrete volume 66 000 m3, LC 60, density 1950 kg/m3 water depth 345 m
1.3 Development of the concrete material When the Ekofisk tank (completed 1973) was designed, the highest strength class allowed according to Norwegian Standard was used, namely B 450 with a cube strength (in present units) of 45 MPa, now denoted C 45. Economy favoured a continuous increase of concrete strength grades, in particular because cylindrical and spherical shapes were preferred. These needs contributed strongly to the development of high strength/high performance concretes. The strength grades in recent structures are, for comparison, about C 80–85. The increase has been made possible by a steadily increasing level of knowledge accumulated through experience and research. (Moksnes and Sandvik, 1996), (Neville and Aïtcin, 1998), (Moksnes and Sandvik, 1998). Important factors contributing to the improvements of concrete qualities are: development of a high strength cement well controlled aggregate grading admixtures, in particular superplasticisers and retarders strict quality assurance procedures
• • • •
The mechanical properties of high-strength concrete differ in many ways from those of traditional concrete. Thus, traditional design procedures for reinforced concrete cannot be extrapolated to new strength classes without a thorough study and relevant modifications. To avoid unnecessary restrictions to the application of high-strength concrete, the extended knowledge must be implemented as rules for high-strength concrete in standards and codes of practice (Section 1.6). 1.4 Design 1.4.1 Preliminary design Offshore concrete platforms are constructed inshore, floated to a deep-water site for deckmating and towed to their operation positions offshore. This construction procedure implies that the structures must be hydrodynamically stable under many different conditions. Moreover, dynamic response is important in temporary stages as well as at the operating stage. Such requirements necessitate that geometrical external shapes as well as weights and rigidities (and hence thicknesses) are reasonably well approximated in the preliminary design, and that the detailed analyses mainly serve to specify ordinary reinforcement and prestressing steel. In the preliminary design, basic understanding of structural mechanics and traditional shell theory, and experience from similar structures play an important role, but computer analyses may be also used in this phase. 1.4.2 Global analysis
described under preliminary analyses above. However, the intersections between the different shell elements introduce irregularities, and the wave loads and other loads introduce various forces in addition to the hydrostatic ones. Such facts call for more advanced methods of analysis. The structural analyses have mainly been based on a linear theory of elasticity, and since the mid-seventies on the use of large finite element programs. The largest finite element calculations may involve more than one million degrees of displacement freedoms and require the use of supercomputers (such as CRAY YMP/464 that has been used for the largest analyses) (Brekke, Åldstedt and Grosch, 1994) (Galbraith, Hodgson and Darby, 1993). 1.4.3 Postprocessing. Dimensioning
The offshore platforms are subjected to a large number of loading conditions during the construction, tow-out, installation, operation and removal phases. Large hydrostatic pressures dominate during deck-mating, while wave, current and wind loads dominate during the operation phase. To permit the handling of all relevant load cases, a number of basic load cases are selected, from which the actual load cases with load factors for the relevant limit state, possible amplification factors, etc; may be obtained by linear scaling and superposition. To utilize the huge amount of data from the finite element analysis in an efficient dimensioning of the reinforced concrete sections of the structure, a post-processor that is specially developed for the purpose is needed (Brekke, Åldstedt and Grosch, 1994). The strength of the reinforced concrete is checked point-wise by comparing the stress resultants with the strength in the same point. The strength evaluation relies on semi-empirical design formulae, mainly based on reduced scale experiments on beams and column elements, and is taking into account cracking and other non-linear effects. The design formulae are specified in codes and standards, but have also been supplemented by special procedures in the post-processors (Brekke, Åldstedt and Grosch, 1994). Refinement of the methods is still going on (Gérin and Adebar, 1998). 1.5 Construction methods Offshore concrete platforms are constructed inshore, and vertical walls have mainly been constructed by slipforming. Slipforming has also been extended to be used for non vertical walls, variable thicknesses and variation of diameters and cross section shapes as usually needed in the shafts. The slipforming method requires a careful control of the concrete consistency in order to avoid flaws in the concrete surfaces, thus requiring an intimate interaction between material technology and construction procedure. When the concrete structure is completed, it is floated to a deep-water site for deck-mating and towed to the operation position offshore. The production hence also includes challenging marine operations in narrow fjords.
1.6 Rules and regulations 1.6.1 Government regulations
Design and construction of offshore structures must, like structures onshore, follow rules that basically are laid down by the government that has the sovereignty of the area in question, e.g. in: USA: United States Department of the Interior UK: Department of Energy: Statutory Instruments SI 289 1974 The offshore installations Norway: Norwegian Petroleum Directorate Norwegian Petroleum Law with Regulations and Guidelines (NPD, latest version applies). For the design work in Norwegian waters the following regulations are of particular relevance: • Regulations relating to safety, etc. to Act No. 11 of March 22nd 1985, relating to the petroleum activities • Regulations relating to loadbearing structures in the petroleum activities including: * Guidelines to regulations * Guidelines concerning loads and load effects * Guidelines relating to concrete structures • Regulations relating to the licensee’s internal control in the petroleum activities on the Norwegian continental shelf • Regulations relating to implementation and use of risk analyses in the petroleum activities, with Guidelines. As for structural concrete, Norwegian Petroleum Directorate’s “Regulations relating to load bearing structures with Guidelines” are mainly based on Norwegian standards; see also Section 1.6.2 and Chapter 5. 1.6.2 Standards
• Norwegian Council for Building Standardisation (1999), Specification texts for building and construction, NS 3420, Oslo, Norway, 2nd edition 1986, 3rd edition 1999. Other documents may play a similar role, e.g. ACI 318–95, saying in the introduction: “The code has been written in such a form that it may be adopted by reference in a general building code...” The European prestandard (Eurocode 2, 1991) covers concrete structures in general, but says explicitly that it does not cover offshore platforms. Standards are in general not mandatory documents. Similarly, they may also be used outside the country or region where they were issued. As an example, the Norwegian standard for concrete structures was used for the concrete platform on the Hibernia field, Newfoundland, Canada. The reason why the Norwegian standard was preferred was mainly that the operator (Mobil) was well acquainted with this standard from previous projects in the North Sea. 1.6.3 Certification. Classification companies
Control and approval of offshore installations is regulated by national government authorities. The third party role of classification societies in this activity differs (Andersen and Collett, 1989). The most active classification societies in offshore activities are Lloyd’s Register and DNV, which may be described briefly as follows: • Lloyd’s Register is the world’s premier ship classification society and a leading independent technical inspection and advisory organisation, operating from more than 260 exclusively staffed offices worldwide and served by 3,900 technical and administrative staff. • Det Norske Veritas (DNV), Oslo is an independent, autonomous foundation established in 1864 with the objective of safeguarding life, property and the environment. DNV has 4,400 employees and 300 offices in 100 countries. DNV establishes rules for the construction of ships and mobile offshore platforms and carries out in-service inspection of ships and mobile offshore units. 1.6.4 Company specifications Codes and standards are often not sufficient as technical contract documents. Thus, oil companies often choose to issue their own, more detailed, company specifications. Such specifications may also prescribe safety requirements in addition to those given in rules and regulations. An example of such a specification is NSD 001, issued by Statoil, a Norwegian oil company. 1.6.5 Development of codes and standards
• fib: International Federation for Structural Concrete (established 1998 by merging FIP and CEB) • ACI: American Concrete Institute • RILEM: International Union of Testing and Research Laboratories for Materials and Structures. 1.7 Quality assurance The highly automated analyses by using finite element methods and dimensioning by postprocessors have their pit-falls. Thus, comprehensive schemes for quality assurance are implemented to avoid errors in analysis and design, including simplified checks of results of the global analysis, mainly equilibrium checks. A manual issued by the Norwegian oil company Statoil recommends that the simplified preliminary analyses discussed above are systemized in such a way as to also serve the purpose of a rough check of the results of the detailed analyses (Gudmestad, Holand, and Jersin, 1996). The need for quality assurance procedures is well illustrated by the Sleipner accident. The gravity base structure of the Sleipner A platform is a traditional Condeep platform placed at a moderate depth of 82 m in the North Sea. The first concrete hull built for this purpose sprang a leak and sank under a controlled ballasting operation in Gandsfjorden outside Stavanger, Norway, on 23 August 1991 (Jakobsen, 1992). It was rebuilt and placed in position in 1993. 1.8 Durability The first concrete platform was placed in the North Sea in 1973. Since then the behaviour of these structures has been investigated thoroughly by means of inspection and instrumentation programmes. In addition, data from maintenance and repair reports are available. Based on such data, the durability of offshore concrete structures has been studied by a working group appointed by FIP (FIP 1996). The conclusions of this group are, directly quoted: • • • • the concrete offshore platforms provide full operational safety they show a very high durability level they do not require costly maintenance and repair operations their effective lifespan has been underestimated and their 20 years initial design life can be greatly protracted
The document has been based on an inquiry answered by: • • • • The Norwegian Petroleum Directorate Oil companies Certifying authorities Contractors and consultants
Similar conclusions are found in (Ågnes, 1997), (Moksnes and Sandvik, 1998), (Bech and Carlsen, 1999) and (Helland and Bjerkeli, 1999). The FIP report also contains recommendations for design, construction and inspection practice. 1.9 Competitiveness In spite of good experience with concrete structures, they will not be competitive for all offshore projects. A few essential arguments for the choice of a concrete structure, because of cost efficiency, are listed below (Ågnes, 1997): • • • • • • • • Topside weight. Heavy topsides can be accomodated on a concrete substructure. Storage. Oil and stable condensate can be stored in concrete cells. Durability and maintenance. Concrete is favourable when long life-time is desired. Seabed conditions. On firm soils the concrete platform rests perfectly by its own weight. On soft soils long skirts provide an efficient solution. Collision strength. Concrete is robust to local damage. Motion characteristics of floaters. Concrete platforms offer better characteristics because of larger displacement. Ice infested areas. Concrete may be designed to resist ice forces. Local content. Large parts of the plain construction work can be carried out by unskilled labour under competent guidance.
Cost competitiveness is also discussed in (Collier, 1997) and (Michel, 1997). Marine concrete structures for the future are discussed by showing several options by Olsen (Olsen, 1998) and by Iorns (Iorns, 1999). 1.10 Removal. Demolition. Recycling
1.11 Spin-off effects The technology developed for the offshore concrete structures has had a number of spin-off effects for onshore or near-shore construction technology. The following know-how and analysis tools for advanced technologies are mentioned, with examples of use for other types of structures: Know-how on: • high performance concrete (sub-merged tunnels, any concrete structures designed for longterm durability) • high-strength normal-weight concrete (long-span bridges) • high-strength lightweight aggregate concrete (long-span bridges, floating bridges) • complex slip-forming with change of thickness and change of cross-section shape (towers, silos) • marine operations in open sea (complex marine transfer and tow operations) • marine operations in coastal waters (floating bridges, submerged tubular bridges) • underwater soil mechanics (submerged tunnels) • evaluation of accidental actions (industrial plants). Software for: • • • • finite element analyses (irregular box-shaped bridges) dynamic analyses of structures (towers, bridges built by cantilevering techniques) static and dynamic wave force analyses (floating bridges, submerged tubular bridges) pre-processors and post-processors for structural design (bridges, other complex structures).
Bech, S. and Carlsen, J.E. (1999) Durability of high-strength offshore concrete structures. 5th International Symposium of High Strength/High Performance Concrete Structures. Eds. Holand, I. and Sellevold, E.J.Sandefjord, Norway, 1999. pp. 1387–1394. Brekke, D.-E., Åldstedt, E. and Grosch, H. (1994) Design of Offshore Concrete Structure Based on Postprocessing of Results from Finite Element Analysis (FEA), Proceedings of the Fourth International Offshore and Polar Engineering Conference, Osaka, Japan. Brown, P. and Nygaard, C. (1997) New Generation TLP. CONCRETE a feasible option for offshore construction. Two-day International Conference, IBC Technical Services, Aberdeen May 1997. Chabot, L. (1997) Spar structures—Steel versus concrete. CONCRETE a feasible option for offshore construction. Two-day International Conference, IBC Technical Services, Aberdeen May 1997. Collier, D. (1997) Cost competitive concrete platforms—Innovative solutions for today’s market. CONCRETE a feasible option for offshore construction. Two-day International Conference, IBC Technical Services, Aberdeen May 1997. Eurocode 2 European Prestandard ENV 1992–1–1. (1991): Design of concrete structures. CEN 1991 (under revision 1999 for transformation to EN, European Standard). FIP (1996). State of the Art Report—Durability of concrete structures in the North Sea. SETO, London. FIP (1998) Awards for Outstanding Structures. XIII FIP Congress 1998, Amsterdam. Fjeld, S. and Morley, C.T. (1983) Offshore concrete structures in Handbook of Structural Concrete. Eds. Kong, F.K., Evans, R.H., Cohen, E. and Roll, F., Pitman, London. Galbraith, D.N., Hodgson, T. and Darby, K. (1993) Beryl Alpha—Condeep GBS Analysis. SPE 26689. Offshore Europe Conference, Aberdeen September 1993. Gérin, M. and Adebar, P. (1998) Filtering analysis output improves the design of concrete structures. Concrete International. December 1998. pp. 21–26. Gudmestad, O.T., Holand, I. and Jersin, E. (1996) Manual for Design of Offshore Concrete Structures. Statoil, Stavanger, Norway. Gudmestad, O.T., Warland, T. Aa. and Stead, B.L. (1993) Concrete Structures for development of offshore fields. Journal of Petroleum Technology, August 1993. pp. 762–770. Helland, S. and Bjerkeli L. (1999) Service life of concrete offshore structures. Offshore West Africa ’99 Conference and Exhibition, Abidjan, Ivory Coast.
Holand, I. and Lenschow, R. (1996) Research Behind the Success of the Concrete Platforms in the North Sea. Mete A. Sozen Symposium. ACI SP-162. Farmington Hills, Michigan, pp. 235–272. Høyland, K. and Maslia, J. (1999) Removal and recycling of high strength offshore concrete structures. 5th International Symposium on Utilization of High Strength/High Performance Concrete. Sandefjord, Norway. Irons, M.E. (1999) Low-Cost Ocean Platform Construction—A Point of view. Concrete International. December 1999. Jakobsen, B. (1992) The Loss of the Sleipner A Platform. Proceedings of the Second (1992) International Offshore and Polar Engineering Conference. San Francisco 1992. Leivestad, S. (1999) ISO Standard for fixed concrete structures. 5th International Symposium of High Strength/High Performance Concrete Structures. Edited by Holand, I and Sellevold, E.J., Sandefjord, Norway, 1999. pp. 421–426. Michel, D. (1997) The advantages of floating concrete construction. CONCRETE a feasible option for offshore construction. Two-day International Conference, IBC Technical Services, Aberdeen May 1997. Moksnes, J. (1990): Oil and Gas Concrete Platforms in the North Sea—Reflections on two Decades of Experience. Durability of Concrete in Marine Environment, An International Symposium Honoring Professor Ben C.Gerwick, Jr., University of California. Moksnes, J. and Sandvik, M. (1996) Offshore concrete structure in the North Sea. A review of 25 years continuous development and practice in concrete technology. Odd E.Gjørv Symposium on concrete for marine structures. New Brunswick, Canada. Moksnes, J. and Sandvik, M. (1998). Offshore concrete in the North Sea—Development and practice in Concrete Technology. Concrete under severe conditions 2. E & FN Spon, London, pp. 2017–2027. Neville, A. and Aïtcin, P.-C. (1998) High performance concrete—An overview. Materials and Structures, Vol. 31, pp.111–117. Norwegian Council for Building Standardisation, NBR (1998), Concrete Structures, Design rules. NS 3473, 4th edition, Oslo, Norway, 1992 (in English), 5th edition 1998 (English Edition in print). Norwegian Council for Building Standardisation, NBR (1999), Specification texts for building and construction”, NS 3420, Oslo, Norway, 2nd edition 1986, 3rd edition 1999. Nygaard, C. (1997) Concrete—A potentially schedule competitive option. CONCRETE a
feasible option for offshore construction. Two-day International Conference, IBC Technical Services, May 1997. O’Flynn, M. (1997) Gravity base structures and jack-up platforms. CONCRETE a feasible option for offshore construction. Two-day International Conference IBC Technical Services, May 1997. Olsen, T.O. and Høyland, K. (1998) Disposal of concrete offshore platforms—Is recycling of materials an acceptable option? Sustainable Construction: Use of Recycled Concrete Aggregate. Thomas Telford, London. Olsen, T.O. (1998) Marine concrete structures. Concrete under severe conditions 2. E & FN Spon, London, pp. 1596–1605. Olsen, T.O. (1999) New generation marine concrete structures. 5th International Symposium of High Strength/High Performance Concrete Structures. Edited by Holand, I. and Sellevold, E.J.Sandefjord, Norway, 1999. pp 91–98. Stead, B.L. and Gudmestad, O.T. (1993) A concrete platform for re-use in variable water depths, with varying topside functions and weights. 1993 OMAE—Vol. 1, Offshore Technology. ASME.
2 Concept definition and project organization
Ove T.Gudmestad, Statoil
2.1 Objectives The objectives of Chapter 2 are to contribute to: • • • • give an overview of the requirements for design of offshore concrete structures. convey the experiences from prior projects, to those having special interest in offshore concrete structures. promote and enhance the confidence in offshore concrete structures. give an overview of how to design a concrete platform, an overview which can also be suitable reading for students.
2.2 General description of an offshore concrete structure Prior to any further discussion regarding design of an offshore concrete structure, references are made to Figures 2.1 and 2.2, which show typical fixed and floating concrete structures, respectively. It is of special importance, for further insight, to recognise the names of the various elements of the structures. For several typical offshore concrete concepts, floating stability is not achieved if one (or more) of the compartments are damaged and flooded with water. This is representing a line of thinking in design which is not common in connection with ship-design. It also means that structural design must be done with particular care. For fixed bottom founded concrete structures the importance of floating stability applies during the floating phases only, as the structures cannot sink after being installed offshore. Floating concrete structures have to be designed with sufficient safety against sinking, in case compartments facing open sea would be filled with water during operations at the field. For design of concrete structures the requirements of Section 18 of the Norwegian Petroleum Directorate’s “Regulations relating to load bearing structures in the petroleum activities” should be given special attention: The structural system, details and components shall be such that the structures: a) b) c) d) e) f) show optimum ductile properties and little sensitivity to local damage are simple to make provide simple stress paths with small stress concentrations are resistant to corrosion and other determinations are suitable for simple condition monitoring, maintenance and repair are removable.
2.3 Project phases During design of an offshore structure it is worthwhile noticing that the work is performed in several project phases with an increasing degree of detail (Fig. 2.3). During the first phase, for example, the advantages of various structures is assessed, and comparisons are made for field developments using various types of structures. As part of the work during the detail design phase, which forms part of the construction phase (not shown in Fig. 2.3), the detailed calculations are made. For concrete structures this includes geometry drawings, rebar drawings, rebar bending schedules, etc. More detailed description of the work in the various phases are given in the following sections; see also Fig. 2.3 and Appendix A.
Fig. 2.3 Project Phases for Design of Marine Structures 2.4 Rules and regulations Offshore concrete structures are to be designed according to national rules and regulations (see Section 1.6 and also (NPD, 1992), (NBR, 1998) and (NBR, 1999).
2.5 Project management 2.5.1 Project planning (a) The objective of project planning Design of an offshore structure should be regarded as a project, i.e. a set of tasks to be accomplished within a specified period of time, and with limited resources. Also, there must be a project organisation with responsibility for execution of the project task. A project is a link in a chain, where the effectiveness and quality, among other things, depend on the interaction between the various links; project employer, project and supplier, Fig. 2.4.
Fig. 2.4 Description of a project as a link in a chain The purpose of the project planning is thus to: • • • • distribute responsibility, authority and tasks achieve high quality of the project results manage resources, time and cost and control the use of them reduce the use of double work and unproductive/unnecessary project tasks.
(b) Control activities To achieve the objectives of the project planning, it is important to establish necessary control activities to ascertain the fulfilment of the objectives (Fig. 2.5).
The control activities are: • • • • • • to establish goals establish an activity plan to reach the goals control the execution of the project in accordance with the plan follow up the execution identify and analyse plan deviation plan and perform improvements and, if necessary, take care of corrective activities.
Design of offshore structures will be a sub-project within a major investment project. An investment project can be characterised by a high exposure of cost, combined with high uncertainties. The uncertainties are partly linked to the investment cost for facilities and partly to future incomes. The development of an investment project will last for years, with several decision points (milestones). The project is therefore sub-divided into project phases as discussed in the previous sections of this chapter. 2.5.2 The project control basis
(a) Introduction The project control basis, Fig. 2.6, can, as a minimum, be defined as: • • • • work scope activity plan (network) with planned progress cost estimate (time distributed costs) authorisation.
The control basis should be compiled before the start of each of the phases in the investment project. In addition, the project control basis should define the control parameters influencing the project objective. The control parameters should be consistent through all the project phases, and should be updated when new information gives grounds for changing the parameters. The result of the planning process: milestones, resource planning and cost phasing establish an execution plan as control basis for the next project phase. (b) Project breakdown structure The project control basis should be broken down according to a standard cost coding system, enabling easier planning and control of the project, such that deviations can be detected and corrective actions implemented. The cost coding system should make allowances for various requirements, depending on the project phase, i.e. if it is in an early planning phase or in a later project phase (execution). The cost coding system is designed such that planning data for various project alternatives can be compared and analysed in all the project phases. The cost coding system will be the foundation for systematically feeding back of experience data and for compilation of time schedules and estimates. The cost coding (Fig. 2.7) accommodates the following three hierarchy structures: • Physical Breakdown Structure • Standard Activity Breakdown • Code of Resource.
Fig. 2.8 Work breakdown structure (AFE= Authorization for Expenditure) The breakdown into work packages should take the following into consideration: • • • • • • organisation and ownership contract philosophy supplier marked availability work complexity interface internally and externally in the project method of assessment and control of workmanship.
The execution plans should include: • • • • scope of work (including technical specifications) progress plans (including externally given milestones) resource plans cost estimates (including budgets).
The relation between scope of work, time, resources and cost are linked to the lowest level (planning level 0) in the project’s Work Breakdown Structure (WBS) see Fig. 2.8. (d) Scope of work The client is responsible for a proper definition of the project’s goals, and to ensure that the goals are understood by all parties involved. The main goal of a project is always to strive for cost/benefit-effect (i.e. to maximise the profit on the invested capital). The correlation between the various sub-goals for the project and the main goals can be difficult to understand. The project control parameters must therefore be clearly defined, to assure that all involved have got a mutually agreed understanding of common goals, project tasks, assumptions/ frame conditions in the entire chain from client to project and to contractor/supplier. Project agreement. The project goal and the overall control parameters shall be documented in a project agreement. The project agreement shall describe goals and tasks, assumptions and frame conditions, plans and estimates, responsibilities and authorities. The document is prepared by the client. Contracts. The need for mutual goals and understanding of project scope, assumptions and frame conditions also applies to the supplier for those parts of the project for which he is being given responsibility. During contract formulation (see also Section 2.8), and following-up of the contract, it must be assured that the project’s requirements to management and control systems is met so that project goals can be reached. By setting contract requirements for quality management and control to contractors/ suppliers, the possibility of preventing negative deviations are increased. (e) Schedule The overall progress plan forming the basis for execution of the project, is called “Master Control Schedule”. During project execution, deviations will occur, hereby creating the need for schedule revisions, called Current Control Schedule. The work packages in the project containing volume, time and cost shall be split into work orders, CTRs (cost, time, resource estimates), by the contractor/supplier. Schedules are normally presented in two ways: A network (Fig. 2.9) containing necessary information about work sequences and logic for the aim of analyses, as well as a Ganttdiagram for presentation purposes of the project, (Fig. 2.12).
Network. The activities’ dependence on each other should be modelled in a project network (Fig. 2.9). The level of detailing and complexity in the network model will be determined by the project’s complexity, magnitude and requirements for quality and follow up. The network definition comprises of: • • • • activity dependence with type of bonding early start/finish late start/finish delays/overlaps.
Fig. 2.9 Project network Analysis and presentation (Gantt-diagram). The final schedule, with built in slack and overlapping activities, should be drawn up and determined from what will overall give the best project economy. The likelihood of meeting the ending date or in-between milestones should also be determined (Fig. 2.10).
The time schedule should be presented in a Gantt chart (Fig. 2.12) with duration, start, finish and slack for each activity. When the work scope of each activity is not clearly stated in the plan, then it should be indicated separately. The activities’ mutual dependence on each other, the network structure and the anticipated use of resources in each activity, should all be well documented. (f) Resource planning Resource planning (Fig. 2.11) and follow up should be formalised in a system. A Code of Resource is to be used for each system where it is considered necessary, with respect to cost estimation, duration analysis and physical progress planning. Guidelines for allocation of resources should be worked out and used for planning, registering and follow up of the physical progress. Each activity’s minimum duration should be defined, together with the required resources and the resulting costs at the same time as constraints from such factors as safety and environmental concerns are satisfied. The use of resources and funds as a function of duration should generally be determined.
Fig. 2.11 Resource planning (g) Cost estimate requirements Cost estimates. The cost estimate is an approximation of the final project costs, based on facts and reasoning. The estimate should be worked out in accordance with the relevant cost coding system for the project phase. Presumptions for a cost estimate, such as: • • • • scope of work/technical solution inflation, exchange rates uncertainty specific planning competence
Specific planning competence is the project foundation, as defined in the project baseline. The estimation method will depend on how many of the four variables: • • • • scope complexity productivity price
are declared. Estimation methods (Fig. 2.13) The estimation method will be selected based on the project phase reached, level of technical definition and access to experience data. During the early project phases when the extent and complexity of technical definition is limited, the synthetic method will be used, i.e. estimation by relations and factors from experience data, main parameters and technical description. The analytic method, i.e. estimation of the all contributing elements directly, where the technical concept is well defined and the scope of work and complexity can be determined, is used in later phases of the project, where the contributing factors can be specified and estimated in detail. When new concept solutions are proposed, the analytic method will also apply for early phases. The analytic method shall always be used for project development, concept definition and project execution phases.
The estimation norms are so set that under given circumstances there should be equal probability for result over as under the individual unit rate (50/50 estimate). An estimate is presented with an expectation value (50/50 estimate, i.e. the value giving the same probability for over/under-run), min/max values and confidence level. All four variables; scope, complexity, productivity and price are related to uncertainties and they should, dependant on method used when estimating, interpretation of available data, etc., be described by a probability distribution (Fig. 2.14). Simplified, this can be a 50/50 value in addition to the low/high values.
Fig. 2.14 Cost estimation uncertainty Requirements for cost estimation and schedule classification. Requirements for cost estimation and time scheduling classification is a classification system with defined requirements to: • • • • • • basic information, work scope estimation method level of detailing time scheduling uncertainties analysis, etc. presentation and documentation formats.
The classification requirements shall describe the method for cost estimation and time scheduling, give requirements to technical information needed to perform the planning and the need for the accuracy of the estimate (Fig. 2.15).
Cost estimates are refined during the course of the project to reflect the additional detail available. A progression of five types of construction cost are normally used; order of magnitude, conceptual, preliminary, definitive and control.
Project budgeting. The budget (Master Control Estimate) is normally equal to the project’s expected cost (50/50-estimate) at the start of the project (Fig. 2.16). The budget is not changed during the execution of the project unless agreed changes between client and project to scope of work or conditions are implemented. Budget changes are always made based on chosen standards of estimation from the original budget.
Phase 2 Project development Feasibility study Objective: Reach a decision whether the actual field is commercial and prepare a Report of Commerciality
Field development study Objective: Describe the best economical solution for field development, and prepare a Plan for Development and Operation (PDO) of the field. In Norway This PDO-document is to be submitted to the authorities for evaluation and for final approval in the Parliament.
During the early phases the possibilities of influencing the development solution and the economical results of the final product is high. Work done in later phases tends more to focus on details, and the extent of documentation increases. It is important to put forward requirements of what efforts are needed to ensure that enough work is performed, such that: • the results satisfy the requirements of the phase in question • the results are sufficient to start the work in the next phase. As a result of this, a Discipline Activity Model, which describes the required extent of work in the various phases of the study, will be useful. A typical Discipline Activity Model for an offshore concrete structure is included in Appendix A. Note that the Appendix defines the need for analysis in the various phases of the project. Furthermore, it defines how the work shall be quality assured. The Appendix also specifies which reports are to be issued during the various phases. In all phases, a technical basis shall be developed to serve as a basis for estimates of costs and plans. The technical basis has to build on realistic information about field parameters. Of particular importance for production are parameters like: • production volume • number of wells, risers and J-tubes for pull-in of production pipes from subsea wells and from other fields • weight and layout of the production plant (topsides) • required storage volume. Furthermore, field specific environmental parameters are important for assessment of the field development solution. Of special importance is information regarding: • • • • • water depth wave height and sea current earthquake conditions (in areas with potential earthquakes) ice conditions (when relevant) geotechnical conditions.
Concrete structures are relevant for quite a number of various field developments. It should be noted that: • fixed concrete structures with short skirts were developed especially for the hard seabed conditions and the large production plants needed for the North Sea during the 1970’s to 1980’s • fixed concrete structures with long skirts were developed especially for the soft soil conditions into and in the Norwegian Trench (Tjelta and others, 1990) • floating concrete structures have been developed over a long period in the 1980’s and 1990’s. A concrete tension leg platform with concrete foundations have been installed at the Heidrun field. For oil production at the Troll field a concrete floater with chain moorings have been installed. 2.6.3 Design of concrete structures in the early phases, examples In the Prospect Evaluation Phase, data gained by experience are used to evaluate the potentials of new blocks. The work can be simplified by the use of PC-based tools. An illustration of input data needed is shown in Fig. 2.17, see also Appendix A (Discipline Activity Model for Design of Offshore Concrete Structures). In the Field Evaluation Phase there is a search for one development solution which shows profitability. Even if detailed technical studies are not performed, it is of importance that the persons performing the job have enough experience such that: • the actual solution is prepared thoroughly to assure that the cost estimate is realistic • the actual solution has not got too many extra reserves built in such that the economical potential is lost and further work is stopped. In the Feasibility Study Phase the technical solutions shall be ranked. Uncertainties in the cost estimates are to be reduced to ± 30%. In this phase concrete structures are compared with alternative solutions. This represents a special challenge to the concrete designers and forces them to establish innovative solutions. Both oil companies, relevant engineering groups and contractors are to be involved. Notice that the development work to obtain a competitive concept is interactive, as shown in Fig. 2.18: • constructive solutions are chosen • load calculations are made and structural analyses are performed • structural design is carried out This is done for the various activities of a development project, including:
construction transportation installation operation removing.
Since the objective of the Feasibility Study Phase is to rank the actual technical solutions, the technical work has to be performed thoroughly. In particular the need for Quality Assurance is emphasised. This is done by engaging highly experienced persons to participate in the studies and to evaluate the technical reports. In the Feasibility Study Phase it is also relevant to consider new concepts with large potential, which so far only have been developed to a lower degree of detail. Special programs for technology development can be initiated to qualify development solutions with much potential. In some countries where the technology development not yet has reached Western European levels, there is particularly large interest in offshore concrete structures, due to the fact that concrete structures can be built by means of: • local resources (sand, cement, rebars etc.) • extensive use of local work force. Furthermore, the construction of concrete structures may lead to: • development of local infrastructure (production of cement and reinforcement) • development of engineering companies and technical know-how. In a Field Development Study the work in the Feasibility Study Phase is carried to a higher detailed level. A more comprehensive technical study is required, to ensure that the cost ~20% accuracy) submitted to the authorities, and which also forms the basis for the estimates (~ authorities’ decisions, are valid also for the successive phases of the development. Appendix A (Discipline Activity Model for Design of Offshore Concrete Structures) indicates the level of detail for technical studies relating to an offshore concrete structure. Special attention shall be made to the fact that an acceptable Plan for Development and Operation (PDO) which is submitted to the authorities after the finalisation of the Field Development Study Phase, requires good insight in: • the environmental loads and other loads like ship collisions, etc. • the geotechnical conditions • the interface between the substructure and the production plant (topsides), including the decision of whether transfer of the topside to the platform shall take place inshore or be lifted on at the offshore site. To ensure that the correct concept is chosen also implies that:
• choice of concrete quality must be made (to assure a robust structure it is suggested that normal density concrete of quality C70 represents the upper limit for this stage) • choice of routing of flowlines from wells, risers and J-tubes. Large risers for gas are generally routed inside a dry shaft or outside the structure until a location below waterline • development of criteria for potential use of jack-up drilling platforms must be made. This decides the layout of the foundation of the concrete structure. Furthermore it shall be noted that tank-testing can be relevant at this stage, to assure the quality and the feasibility of the structure, see also NPD’s “Regulations relating to loadbearing structures”, paragraph 30. In case the PDO-report does not open up for choices of concrete structures, it will normally not be relevant to suggest that concrete structures should be chosen in the later phases of the field development. 2.7 The concept definition phase 2.7.1 Concept report
risk of falling objects risk of uncontrolled ingress of water due to pipe failure or malfunctions of the ballast system risk of collisions risks during towing and installation.
In addition, the accidental design loads applied in the operation phase are to be determined. 2.7.4 Schedules and budgets An important part of the concept definition phase is to establish detailed schedules and budgets for the development project. 2.7.5 Competence requirements The concept report represents the final basis for a successful performance of a development project. Consequently, it is of the highest importance that all parties involved use their most experienced engineers and make them responsible for the project’s concept evaluation phase. 2.8 Project organization phase 2.8.1 Introduction The project Organisation Phase represents an intermediate phase inbetween the phases of concept definition and construction. Note that detail design is part of the construction phase. During the project organisation phase the construction work (fabrication) is prepared. Several fundamental principles of technical nature have therefore to be clarified during this phase. 2.8.2 Contract A contract for the construction phase has to be established in this phase. Included herein is choice of type of contract. The following definitions can be used for important tasks during the construction phase: E P C I Engineering work (detail design) Procurement Construction Installation
This does, however, not mean that safety margins shall be applied beyond the requirements of the national rules and standards. Arrangements must be laid down in the contract to ensure that all changes to the concept report shall be agreed by the oil company and treated according to specially agreed procedures. During contract preparation and negotiations it is important that the oil company’s project group has high technical expertise and long administrative experience. 2.8.3 Quality assurance plan
During the Project Organisation Phase the oil company must prepare a plan for how to perform quality assurance of the work during detail engineering and construction. As to procedures and some available tools for quality assurance, see Chapter 6. 2.8.4 Verification plan A plan for verification has to be developed. The plan shall include: • description of the extent of work for the verification, emphasising special critical areas of the structure • requirements to the oil company’s own verification of contractor’s work, with description of the follow-up work during the detail design and construction activities • requirements to the contractors’ internal quality control and surveillance • working methods and work description valid for the consultant performing the third party verification • requirements for prompt implementation of the results from the verification. The choice of strategy for external verification is of special importance. The oil company must have direct contact with the third party verification consultant. Payment should be made according to spent hours within agreed limitations. Chapter 7 details two models for external verification: 1. the oil company submits the results from the verifying consultant to the contractor 2. company is managing the verification, but the verification consultant communicates mainly directly with the contractor. Regardless of which model is followed, the verifying consultant is obliged to follow a tight schedule not to delay the progress of the contractor. The oil company must take all formal decisions in case disagreements arise between the contractor and the third party verificator. The oil company should also be the single point contact with the authorities and the partners.
2.8.5 Preparation for the organization of the construction phase During the process of preparing for a development project, project organisations are built up within the oil company and the contractor. The oil company is to ensure that both his own personnel and those of the contractor have the acceptable competence. This is partly done by specifying competence requirements for every leading position in the project. Every leader must have thorough knowledge of the technical content of the activity he is meant to lead. It is presupposed that the project is organised to assure openness with respect to technical questions, and that the requirements for quality in performance is characterising the dialogues and feed backs. No project philosophy which suppresses technical problems should be allowed In offshore field development projects, key persons who have worked during earlier phases in the same project should be brought in, so that continuity in the technical work is maintained. All staff in a project must agree to the project’s goals and the procedures applied to reach these goals. References Fjeld, S. and Morely, C.T. (1983) Offshore Concrete Structures, in Handbook of Structural Concrete. Eds Kong, F.K., Evans, R.H., Cohen, E. and Roll, R, McGraw Hill. Norwegian Council for Building Standardisation, NBR (1999) Specification texts for building and construction, NS 3420, Oslo, Norway, 2nd edition 1986, 3rd edition 1999. Norwegian Council for Building Standardisation, NBR (1998), Concrete Structures, Design rules. NS 3473, 4th edition, Oslo, Norway, 1992 (in English), 5th edition 1998 (English edition in print). Norwegian Petroleum Directorate, NPD, Rules and Regulations for Petroleum Activities, New edition issued every year by the Norwegian Petroleum Directorate, Stavanger, Norway. Norwegian Petroleum Directorate, NPD (1992) Regulations relating to loadbearing structures in the petroleum activities, stipulated by the Norwegian Petroleum Directorate, Stavanger, Norway. Tjelta, T.I., Aas, P.M., Hermstad, J. and Andenæs, E. (1990) The skirt piled Gullfaks C platform installation. Paper OTC 6473. Proceedings Offshore Technology Conference, pp. 453–462, Houston, Texas.
3 Simplified analyses
Tore H.Søreide, Reinertsen Engineering
3.1 Introduction This Chapter deals with simplified calculation schemes for use in the engineering of marine structures, as an alternative to complex computerized techniques for response analysis. The main objective of the development of analytical techniques is to come up with a tool for early estimates of dimensions, prior to the start of the process of detail engineering. Furthermore, simplified methods, either by hand or based on spreadsheets, are also useful for the control of the complex scheme of global response analysis. The complete analysis set-up is shown below in Section 3.2, which demonstrates that for simplified response analysis there is a need both for global and local models for capacity control. As a basis for the evaluation of calculation methods, the loads are to be classified in accordance with the characteristics of their impact on the structural system. Section 3.3 gives a brief introduction into basic dynamics, which is also relevant when deciding the type of analysis in the global response model. Section 3.4 presents simplified analytical techniques to be used for fixed gravity based platforms. The global response is calculated by modal techniques which keep the number of parameters to a minimum. The analysis schemes for floating marine structures are given in Section 3.5, where catenary anchored as well as tension leg platforms are dealt with. Formulas are depicted for the analysis of first order wave effects, ringing effects as well as hydrostatic stability. Section 3.6 considers ship impact and presents methods for global response analysis. Section 3.7 handles second order geometric effects in design, including shafts and planar walls as well as cylindrical cell walls. The geometric effects from finite rigid body rotations of floating structures are also illustrated. The problem areas dealt with for floating marine structures are also relevant for fixed structures during fabrication, tow and installation, especially the hydrostatic stability calculations, built-in forces and skew ballast. 3.2 Analysis activities 3.2.1 Analysis for detail design
3.3 Classification of load effects 3.3.1 System analysis Prior to the activity on final analysis models, a system analysis is to be made as a basis for the subsequent selection of analysis models. The main objective of this evaluation of response characteristics is to sort load effects into respectively static and dynamic classes of response. The characteristics of structural changes during fabrication and installation and the evaluation of load effects must produce relevant response models for all stages. It is convenient to separate the displacements of the system into rigid body motion and deformation modes, respectively. The system analysis is to be documented. Experience from larger projects proves that all personnel involved in the engineering team benefit from a presentation of the system analysis as a basis for their considerations concerning design load situations. A description of the structural load carrying behaviour also makes the control of the global analysis results easier for the engineering personnel. 3.3.2 Load effects
Once the structural system is determined, the different loads are to be categorized in accordance with their influence on the structure. This will reveal if static response can be applied, or a dynamic model is needed. As a basis for this evaluation of response characteristics, the natural frequencies of the system should be made available, either by an element analysis, or alternatively, from simple analytical estimates. As an example of categorization of loads, reference is made to the Norwegian Petroleum Directorate (NPD) regulations, see Section 1.6.1. The following load types normally imply static response analysis: • • • • • • • • • Dead load (permanent load) Ballast (variable load) Prestress Hydrostatic pressure (permanent load) Tide (variable load) Current (variable load) Mean 10-min. wind (variable load) Built-in forces (permanent load) Imposed deformations (deformation load, temperature, shrinkage).
For the cases of wave load response and for impacts, dynamic effects are to be included. Dynamic wave analysis also implies the consideration of the fabrication stages, for which the deformation modes may be flexible with low natural frequencies close to the highest wave frequencies (0.5–0.2 Hz).
The upper and lower outer limits for the dynamic response characteristics are as follows: Stiffness dominated The frequency of the load is low when compared to the modal frequencies of the structure: KX= R(t) Inertia effects are neglected, as for a fixed gravity base platform in a permanent situation. Inertia dominated The load frequency is high when compared to the natural frequencies, resulting in dominance from inertia forces in the dynamic equilibrium: ¨ =R(t) MX (3.2) (3.1)
This is often the situation with impact loads. For a catenary anchored floater, all six rigid body motions may be inertia dominated, while for a tension leg platform, the surge and sway directions of motion are inertia dominated. Between the two above outer limits, stiffness, mass and damping govern the load effects. For local stress control, static analysis can normally be used. 3.4 Gravity base structures 3.4.1 Model for global response analysis
This section deals with a simplified model for global response analysis of gravity base concrete structures, which combines the contribution of the rigid element displacement of the structure and the beam effect of the shafts. The actual load effects are displacement and acceleration of the deck, and alternatively, beam moment and shear at the base of the shaft. Fig. 3.4 points to several factors that need to be evaluated prior to running the global analysis model. These factors include: stiffness characteristics, mass motion and soil damping, as well as the deformation characteristics of the caisson cell walls that influence the degree of clamping at the base of shaft. The deck connection to the top of shafts affects also the moment and the shear force in the shafts. A total understanding of load distribution in the deck and the structure is necessary. Interaction with the surrounding water shall be accounted for. Fig. 3.5 gives an illustration of load, mass and damping that are included in the dynamic model.
Fig. 3.4 Simplified model There are now two ways to perform a simplified global analysis: either by a simplified element model (beam elements for shafts and shell elements in the caisson), or by hand calculations. The following procedure gives a rough indication of these approaches: A. B. C. D. The analyses should include all critical phases such as construction, towing, installation and operation. For each phase an eigenvalue analysis is performed with due consideration of water mass. If the natural period of the structure is substantially below the load period interval, a quasi-static calculation is performed by neglecting the mass contribution. When masses are believed to influence the behaviour of a slender structure they are taken into account.
Fig. 3.6 shows a model of the submerged phase just prior to coupling the deck to the structure and also a model in the operation phase. In both models the shafts are assumed to be clamped at the top of the caisson. For the submerged phase, it is necessary to compare the natural frequency with the wave period at the actual location. In protected water, such as in a fjord, the values are somewhere between 2 and 6 seconds. Clement weather is needed for coupling operations. In the operation phase the composite action of the deck and the structure is considered. In Fig. 3.6b a simply supported connection is indicated between the deck and the shaft (situation just after coupling). A fixed connection is then established by stressing cables and grouting the space between the deck panels and the shaft. An indication of the deck stiffness compared to the shaft stiffness can be obtained by calculating the modal stiffness in the same mode for the deck and the shaft. In Fig. 3.7 an alternative global model is shown, where the caisson is assumed to be stiff, but the stiffness, the mass and the damping are included for the ground. The general equations for the elements in modal stiffness, mass and damping are:
(3.6) (3.7) (3.8) where: ms ma Ck ca = = = = mass of structure including ballast water additional mass of surrounding water damping in structure damping from surrounding water.
In relation to equations in Fig. 3.6, Fig. 3.7 includes rigid body displacement of shafts and caisson. It is appropriate to use a two-degrees of freedom system, where the modal amplitudes are, for example, the rotation of the caisson and the horizontal displacement of the top of shafts. The displacement pattern with two degrees of freedom may be described by the global modes (see Fig. 3.8): u(x) = · y for the caisson for the shafts
Horizontal displacement: u(x)= · (x)+␦ · ␦(x) where (x) and ␦(x) are the modal functions. From this comes a 2 x 2 system of stiffness, mass and damping. Two eigenvalues are obtained from the condition det (K–2M)=0 and the equivalent eigenvector from (K-i2 M) Xi=0 (i=1,2) (3.18) (3.17) (3.16)
For a Condeep gravity base structure simplified calculations can be performed for most of the structural parts using shell theories. With reference to Fig. 3.11 this applies to the shaft walls, upper domes, cell walls and lower domes. Common to all the parts is that the dominant load consists of compressive normal forces.
Fig. 3.11 Load effects for a simplified analysis (a) Outer cells For the outer parts of the lower and upper domes and outer cylinder wall of an outer cell, an axisymmetric assumption gives a good indication of the sectional forces. Such analyses can be made by hand, by satisfying the compatibility and equilibrium conditions of the domes, ring beam and cylinder wall on rotation/moment and displacement/shear force. Another method is to use an element program modelling the cell and the domes axisymmetrically. With regard to the choice of element size and type see Chapter 4 that deals specifically with cylindrical shells.
In the unique case of no rotation or displacement at the ring beam, bending of the cylinder wall is characterized by:
where r t v p le = = = = = middle radius of cylinder wall wall thickness Poisson’s ratio outer hydrostatic pressure (uniform) elastic length.
Within a distance of le from each edge, the compression force will be determined from a membrane solution N = p · r (3.22)
An indication of the local bending effect close to the ring beam can be calculated using Equation (3.19) with a wall thickness of t=0.60 m and a middle radius of 10 m with a corresponding elastic length of 1.90 m. For thick cylinder walls the above equations must be adjusted to account for the difference between the middle radius and the radius of surface where the load is applied. (b) Inner cells For the inner cells, the geometry and the boundary conditions are more complex. Apart from the circular walls, some of the caissons also have straight walls. A horizontal beam analogy is more appropriate for those walls when subjected to one-sided hydrostatic pressure. For the upper and lower domes on the other hand, an axisymmetric solution gives appropriate sectional forces when the dome is subjected to hydrostatic pressure. Because of the stiffening effect of the surrounding cells it is a good approximation to assume clamping at the ring beam, in other words no rotation or translation.
(c) Shafts For the shaft the axisymmetric theory can be used to estimate the bending moment and shear at the junction caisson/shaft. Furthermore, the compressive normal force along the shaft can be calculated from (3.22) with correction for thick shell. At the boundary condition caisson/shaft, a special analysis is needed to take into account the effect of the upper domes and the surrounding cells. A local element model by shell or solid elements would be necessary. Forces from earlier global analysis are applied in the model. During coupling of deck to the shaft, large concentrated forces develop. A finite element mesh would be required at the interface to evaluate the forces. In these areas a strut and tie method of calculation can be useful for design. (d) Castings It is not unusual that during detailed design, changes and adjustments occur to the geometry which then does not correspond to the global finite element model. An example is concrete castings around mechanical outfittings. These castings affect the structure, but they are not taken into account in the global analysis. For such situations a local element model is advisable. Displacements from the global analysis are applied to the local model. In addition to castings inside the structure, similar castings are used outside the platform such as around J-tubes and crane footings. 3.5 Floating structures 3.5.1 General This section reviews the major load effects acting on a floating concrete structure. The description is based on simple models for hand calculations or desk computers. The action between the raft and the deck is especially important for a floating structure. This section will nevertheless deal mainly with the concrete raft and takes into account the effect of deck stiffness at the intersection points. With regard to the mass of the deck it is also important that, for the hand calculations, the actual weight distribution and weight placement are substituted. A system analysis with emphasis on global response in the construction and operation phases is considered. The hydrostatic stability calculations are reviewed in addition to the static loading. The static and dynamic characteristics are analysed together with critical deformation patterns of the raft. This gives an overall indication of inertia forces in the dynamic models including the water mass. Catenary anchored platforms as well as tension leg platforms are considered. 3.5.2 System description
As shown in Fig. 3.12, before placement of the deck, the raft has little global bending stiffness. In the submerged phase, such as during mating, the additional mass from the sea is substantial. This gives a high natural period (approximately 3 seconds) that could give a resonance effect for short fjord waves. For maximum depth of submergence, the line of action of the wave is highest. The effects mentioned above can govern the design of the entire raft, not only due to the high hydrostatic pressure but also due to the global effect. Fig. 3.13 shows the total picture of the structure system with the deck included. The deck stiffens the raft at the top and cancels the deformations shown in Fig. 3.12. Load distribution between the raft and the deck is important in the design of the deck as well as the raft. For floating structures where large motions are common, it is more appropriate to split the load effect from displacement and deformations in two, namely one set of rigid body modes superimposed with a set of deformation modes. This is a well established method for the analysis of structures with large motions. Rigid body modes are dominant in the determination of the inertia forces, while the deformation modes determine the sectional forces in the structure.
Fig. 3.12 Raft before deck mating For static loads, the split mentioned above is easy since the calculation of load effects follows the same principle for all modes with regard to rigid body and deformation types. For both mode types a modal stiffness is calculated which is then compared with the modal load. For dynamic loads, the dynamic increase of each individual mode must be included. It is necessary here to have a model where the contribution of the mass from the structure and the
surrounding water is also included. The inertia forces will have a substantial effect on both the rigid body motion and the deformation of the raft. The splitting into rigid body modes and deformation modes is illustrated in Fig. 3.14 for a tension leg platform (TLP). The global stiffnesses which are associated with the rigid body modes are determined from prestressing and material stiffness in the anchorage system as well as from the raft surface water area. For the vertical modes (heave, roll and pitch) the axial stiffness of the tethers of a TLP will, for example, be more dominant than the effect of the surface water area. For a catenary anchored structure the opposite is true. This has an influence on the natural period for the heave and roll where the two types of platform differ.
3.5.3 Global static analysis (a) Hydrostatic stability In Fig. 3.15, the terms needed for hydrostatic stability control are shown, where: TR T G Iw = = = = riser tension anchorage tension weight of platform including water ballast waterline moment of inertia.
For a unit rotation (1 rad), the centre of buoyancy moves a distance eB from the centre line.
(3.23)
where g ٌ = = = specific weight of water (1.027 t/m3) gravity acceleration (9.81 m/s2) displacement (kN).
Moment equilibrium for a unit rotation relative to the anchorage level of the raft (notation see also Fig. 3.15) M = ٌ · (zB + eB - zo) - G · (zG - zo) + ⌺⌬T · a - TR · (zR-Zo) (3.24)
In (3.24) ⌬T is the change in tension of the anchorage lines for a unit rotation, expressed for a TLP as (3.25)
The Z0 requirement is the stability criterion for a free-floating structure. For a TLP in operation the tethers are the dominant items for hydrostatic stability.
(b) Static rigid body motion The raft is assumed now as non-deformable and that the stiffness of the anchorage system determines the motion for given loads. As shown in Fig. 3.16 it is convenient to establish the equilibrium equations in the Cartesian co-ordinate system with the origin located in the centre of the anchorage system of the platform. A 6 x 6 stiffness relation is then established:
(3.28)
For a TLP the stiffness in the horizontal modes can be defined from the geometric stiffness
(3.29)
of the tethers: where Ti is the pretension and Li length of tether number “i”. The vertical stiffness in heave and roll will depend on the axial stiffness of the tethers, whereas the contribution of the water surface area is secondary. We get:
(3.30)
where EA/L is the axial stiffness of a tether and Aw the water surface area of the platform. The expression in (3.28) is linear and does not take into account that the tensile forces in the anchorages change as a consequence of the platform motion. For a TLP where the stiffness of the tethers is dominant, the raft will follow an approximately circular path in the vertical plane for sideways motion. For a given horizontal displacement U, the increase in depth will be:
This increase in depth can be in the order of a few metres. This gives a buoyant force that, once more, alters the force in the tether.
Fig. 3.16 Global reference system
(c) Static deformation of raft This section deals with the deformation mode of the raft under the influence of static loads in order to obtain the sectional forces in the structure. The deformation pattern shown in Fig. 3.17 is a typical example for loads from waves and wind.
Fig. 3.17 Bending deformation of raft To obtain the deformations and the sectional forces one needs the stiffness of the pontoons, the shafts and the deck. It is also important to get the correct stiffness model of the boundaries between the pontoon and the shafts and between the shafts and the deck. A method using hand calculations to obtain the sectional forces is shown below. An alternative method is to use a beam model in an element program, particularly when different load situations are to be analysed. With reference to Fig. 3.18, the virtual deformation figure is chosen with the intention of calculating the moments at the boundaries of the pontoon. It is assumed here that we still have a simply supported connection between the top of the shaft and the deck, this assumption needs to be re-evaluated for each load situation. Given the pontoon moment as MPON, and ␦ as the virtual angle at a section with MPON, the internal virtual work, including two pontoons, is:
For loads parallel to the pontoon (0° or 90°): The static load resultant from wind and/or current is assumed to act with a magnitude P at a height Zp relative to the platform co-ordinate system. It is also assumed that due to P, the change of the anchorage force will be ⌬T for each shaft. For a TLP where the axial stiffness is more dominant, then: (3.33) The outer virtual work is given as: (3.34)
See also Fig. 3.18 for notations. The relation between ␦␣ and ␦ is given from the geometry of Fig. 3.18: (3.35)
By comparing (5.10, 5.12 and 5.13), the bending moment at the boundary of the pontoon is:
(3.36)
The relation above is evolved for a static load parallel to the actual pontoon. For a different load direction it is advantageous to use a simple frame program. The moment diagram over the pontoon for the case shown in Fig. 3.18 would be linear with tension on the opposite side of the two pontoons and nil bending moment at the centre of the pontoon. The shear force from wind/current is constant along the pontoon. By using the virtual work as mentioned above, it is vital to include all the loads contributing to the global equilibrium model, refer to contribution of ⌬T. (d) Residual sectional forces Eccentric positioning of the deck load during the mating operation generally leads to the need for jacking operations to reduce the deformations and the sectional forces in the flexible raft; see Fig. 3.12. This and the simultaneous deballasting of the raft after mating leads to permanent stresses in the raft which must be accounted for in design. The residual sectional forces can be calculated by hand or alternatively by using a beam model in a computer. In this case, it is necessary to model the shaft/deck connection with a finite element model more accurately than is possible with hand calculations. A local shell element model combined with a beam model of the lower part of the raft can be used. (e) Uneven ballast Fig. 3.19 illustrates the situation with uneven ballasting for a doubly symmetrical raft. Two diagonally opposite shafts have more ballast water whereas the two others have correspondingly less. The total ballast and the draft are therefore correct, but the internal redistribution introduces stresses in the raft. By having the modal amplitude as the difference in the deflection between the two sets of shafts, the stiffness of the model would be:
(3.37) where EIPON = LPON = sectional stiffness about the horizontal axis of the pontoon effective length of the pontoon, distance between the boundaries.
For a catenary anchored structure, the raft will carry the entire uneven load. For a TLP on the other hand, a portion of the load will be carried by the tether axial stiffness. To find the relative distribution between the tethers and the raft, a comparison is made between the raft modal stiffness in (3.37) and the modal stiffness of the tethers. (3.38) where EATETH = sectional stiffness of tethers for each shaft
For a TLP, the raft will carry most of the uneven load since the modal stiffness in (3.37) is 10 to 20 times larger than the contribution of the tethers (3.38). The bending moment at the end of the pontoon is (3.39) where P = the additional ballast per shaft. From (3.39) the bending stresses in the pontoons can be calculated. 3.5.4 Global natural period
(3.40) For distribution of masses the following notations are used: mCOL = maCOL = mPON = maPON = construction mass per unit height of column (shaft), ballast included additional mass from surrounding water per unit height of column (shaft) = Cm x g A construction mass per unit length of a pontoon, ballast included additional mass from surrounding water per unit length of pontoon = Cm x g A.
With regard to the deformation mode in Fig. 3.12, the modal mass can be written
(3.41)
In (3.41) the lengths LCOL , LWET and LPON are related to the centre axes. An estimate of the natural period for the chosen mode is: (3.42)
The horizontal natural periods are determined from the lateral stiffness of the anchorage lines, while the water surface area and the axial stiffness of the lines contribute to the vertical stiffness. For a TLP the axial contribution is more dominant; see Section 3.5.3. For rotational modes the illustration in Fig. 3.20 is used. Since the lateral anchorage forces are small, the centre of rotation in the vertical direction will be dependent on the inertia forces. The actual masses are added in the figure. A centre of rotation is chosen with a vertical ordinate Zo. The position of the centre of rotation Zo is chosen such that the horizontal resultant of the mass forces is zero. After that, the mass inertia moment, or the modal mass, is determined.
3.5.5 First order wave This section shows the use of deterministic wave theory to give an indication of the sectional forces in the raft from the first order wave. (a) Loads The relation between maximum wave height and period for the actual field location is assumed to be supplied for ULS (100 year return period) and for SLS (1 year). Fig. 3.21 illustrates such a relation.
Morison’s formula gives two contributions to the load on the raft. The viscous part given as: (3.43) where CD vr A = = = = specific gravity of seawater (1.027 t/m3) viscosity coefficient relative velocity between wave particles and raft area across the velocity, per linear metre.
The other contribution in Morison’s formula is the mass component FM = (1 + Cm) · · V · w where Cm V w = = = coefficient for added mass submerged volume per linear metre wave particle acceleration. (3.44)
As is shown below, the mass component (3.44) is generally the most dominant. From (3.43) and (3.44) we notice that the contribution is 90° out of phase. Distributed load at the waterline per metre length of a single shaft from the mass component in Morison’s formula is then: (3.45) where D T H = = = outer shaft diameter wave period maximum wave height from Fig. 3.21.
The variation of load intensity as a function of water depth z from the waterline is FM (Z)= FMO · e-kz where k g = = wave number gravity acceleration (3.46)
Resultant at depth do below sea water level is (3.49) For short waves, approximate values are used (3.50)
(3.51) Equations (3.48) to (3.51) are used later in the global equilibrium consideration. For the evaluation of critical wave periods, it is interesting to look at the load resultant P as a function of the wave period. The relation H/T in Fig. 3.21 is applied in (3.45) together with the actual outer diameter, specific density of water, as well as a factor for added mass. Since the mass contribution is the most dominant and is 90° out of phase with the viscous term, only (3.45) and (3.50) are applied. (b) Waves in 0 and 90 degree directions Waves in 0 and 90 degree directions act along the main axes of the platform; that is; parallel to or normal to the pontoons. For the bending moment in the pontoon close to the column about the horizontal axis, the case illustrated in Fig. 3.22 often governs the design, with a wave length between the shafts. The situation in Fig. 3.22 is also often critical for the shear at the pontoon ends. All four shafts now have their maximum horizontal wave load in one direction at the same instant, implying horizontal acceleration of the platform Fig. 3.18 is used together with the formulas in (3.3.2–3.3.6) in order to calculate the bending moment in the pontoon. The difference from the static expressions is now that the horizontal inertia force resultant is included, as indicated in Fig. 3.22. As horizontal motion is inertia dominated, for a catenary anchored platform as well as for a TLP, Equation (3.36) for static load effect can be used directly, modifying the vertical arm z to be the distance between wave resultant load and the inertia force, added mass included. P is the resulting wave load on to the raft.
Letting L (m) be the centre spacing of the shafts, the wave period T(s) representing the above situation comes out as: T=0.80 · L0.5 In most cases the wave (3.52) is the shortest design wave for the structure. (c) Diagonal wave For a bending moment in a pontoon about a vertical axis, a diagonal wave is often dominent. In the case of a quadratic floater with L (m) being the centre shaft spacing, the critical wavelength comes out as twice the diagonal spacing, with wave period: T=1.35· L0.5 (3.53) (3.52)
Fig. 3.23 illustrates the mode of deformation resulting in a bending of the pontoons about the horizontal and vertical axes, as well as torsion in the pontoons. Given the modal deformation pattern in Fig. 3.23 the modal contributions to stiffness from bending and torsion can be calculated. In most cases bending of the pontoons about the vertical axis gives by far the dominant terms. Based on the deformation pattern in Fig. 3.23 the bending and torsion moments in the pontoons are calculated after the deformation amplitude has been found from the stiffness expressions. The load situation in Fig. 3.23 for the wave phases of 0 and 180 degrees creates no resulting horizontal load on the structure, and inertia forces do not enter into the expressions. Other wave phases are also to be considered, where platform rigid body motion is to be included.
3.6 Ship impact 3.6.1 General The present section outlines simplified calculation schemes for the response due to ship impact. Major effort has been placed on techniques for calculating global responses, dynamic effects included. The objective of presenting the simplified impact considerations below, is among other things enabling estimates of section forces from impact to be made, and by comparing these to other load effects, evaluate whether ship impact is the governing force. Impact analysis is not part of the automised design scheme and thus requires special analyses. As a supplement to the global response, a complete analysis of impact also includes punching control. Here, the distributed load intensity from impact is the governing parameter, rather than the resultant load. Thus, hard impacts from smaller ships are often critical for punching. Methods for impact analysis of floating and gravity base structures are given below. The illustrations are made for floating structures, so as to see the variation in dynamic effects for different modes of motion, Gravity base structures follow the techniques in Section 3.6.5 for rotation, where the eigenperiod is in the range of the duration of impact. 3.6.2 Impact load
As shown in Fig. 3.24 a central impact is assumed, so that the impact force is directed through the vertical line in the platform mass centre, added mass from water included. The velocity of the ship prior to impact is denoted vo, mass of ship with added mass included is Ms, and platform mass with surrounding water is Mp. Further, it is assumed that the impact force is constant during the duration of the impact, and given by the plastic capacity of the ship. This is an approximation, since for plastic deformation of the ship hull the contact force varies with the indentation. In subsequent expressions the impact force is denoted P (kN) and the duration of impact t (s). 3.6.3 Impact mechanics
In Fig. 3.25 a diagonal impact is suggested towards one of the platform shafts. The load resultant P acts in the waterline area. The global reference system lies at the elevation of the anchorages, with the X-axis directed along the impact force. The impact force has elevation h above the origin of co-ordinates.
Fig. 3.25 Diagonal central impact The two-parameter system for analysis now has X as horizontal translation and R for rotation about the global Y-axis. The following notation is used for stiffness and mass elements: KXX KRR MXX MRR MXR = = = = = anchoring stiffness in translation rotation stiffness about origo due to anchoring and waterline area translation mass from platform and added mass rotation mass about origo of platform and added mass mass coupling since the mass centre lies outside origo.
Below, Equations (3.54) and (3.55) are implemented in modes of response with different dynamic characteristics. The objective of the calculations is now to determine accelerations and inertia forces due to impact, so that sectional forces can be found. 3.6.4 Catenary anchored floater
For a floater, the motions in the horizontal plane in the form of X-translation (surge) and rotation about vertical axis Z (yaw) are inertia dominated, so that according to (3.2) the stiffness terms are neglected. It is now convenient to refer the displacements to the centre of mass for the system, added mass included. Fig. 3.25 depicts the symbols for translation and rotation directions of motion. For a catenary anchored floater the motion in roll or pitch is also inertia dominated. Equation (3.55) for the platform motion now becomes uncoupled also in terms of mass: Translation: MP · ü = P , (0 Յ t Յ t1) ¨ = P · d , (0 Յ t Յ t1) Rotation: IMP · where MP IMP d t t1 = = = = = translation mass of the platform, added mass included inertia moment of the platform, added mass included distance from mass centre to impact force resultant time impact duration. (3.56)
(3.57)
The ship and platform common velocity during impact is thereby:
(3.60) From (3.58–3.60) the duration of impact can be calculated as the time when ship and platform have the common velocity . + d · . , (t = t ) vs = u 1 Impact duration: (3.62) (3.61)
The duration of impact t1 will normally be in the range 0.5–2.0 seconds. An estimate of the platform indentation in the ship hull is now obtained from the displacements: (3.63)
Regarding sectional forces in the raft, the situation during impact is governed by 0 Յ t Յ t1. The global structure behaviour is in the form of frame response effects in the raft. This is easily calculated by hand as in Section 3.5.3, where impact force and inertia forces are included. Alternatively, a frame program is used. 3.6.5 Tension leg platform For a TLP the eigenperiod in rotation ⌰ about the horizontal axis is in the range 2.0–4.0 s, and stiffness and mass determine response in this mode. Dynamic amplification of the response, as related to static stiffness dominated reactions, can take place. The horizontal displacement in surge u is inertia dominated, also for a TLP. From the above, the tension by platform produces different dynamic characteristics for impact in the two modes of motion. As for the catenary anchored floater the stiffness KXX in (6.1) is neglected, and only inertia terms apply. Referring to the mass centre, the parameters u and in coupling terms in stiffness are neglected. Referred to the mass centre, added mass included, the two dynamic equations read Translation : MP · ü=P ¨ =P · d Rotation : K⌰⌰ · ⌰+IMP · ⌰ (3.65)
(3.66)
Equations (3.65) and (3.66) now substitute (3.56) and (3.57) for a catenary anchored floater. The equation of motion (3.55) for the ship is still valid. Implementing the initial conditions: ⌰ = 0 for t = 0 (3.67)
¨ =0 for t=0 ⌰ Equation (3.66) is solved
(3.68)
(3.69)
with K⌰⌰ being the rotation stiffness, dominated by the tethers.
Referring to Fig. 3.15, the rotation stiffness reads:
(3.70) where EAi /Li is axial stiffness of a single tether. The second contribution to K⌰⌰ comes from the waterline area: (3.71) In (3.71) four shafts are assumed with distance c from the platform centre. Again, (3.71) can be neglected in practical design. Equation (3.69) includes the eigenfrequency (rad per sec.) for rotation. As related to static solution, (3.69) comes out with a dynamic amplification: DAF=(1 - cos t) The extreme value of (3.72) is (3.73) where T is the eigenperiod. A dynamic amplification of 2.0 for the rotation mode seems reasonable. The estimation of impact duration t 1 follows the procedure (3.58)—(3.62). For simplification, rotation terms may be neglected for the TLP. Fig. 3.27 illustrates a typical time history for rotation ⌰ in the case of impact duration t1 equal to half the eigenperiod T. For global response in the raft, the instant t = 0, t = t1- and t = t1+ are to be controlled, where t1- is the time just before the end of impact, and t1+ is correspondingly just after the impact. The reaction forces in the tethers give major contributions to retardation as soon as the impact force disappears. The time history in Fig. 3.27 is also typical for the response in the shafts of a gravity base platform. (3.72)
3.7 Non-linear geometric effects 3.7.1 General This section deals with the incremental forces as a consequence of second order geometric effects. In Section 3.7.2 the basis for the second order bending moment on beams is described. The objective is to illustrate simplified rules for calculating additional moments in rules and standards (in e.g. (Eurocode 2, 1991), Section 4.3.5; (ACI 318–95, 1995), Sections 10.11– 10.13; (NS 3473, 1998), Section A.12.2). The formulas in Section 3.7.2 deal with the reduction of global stiffness of the shafts of a gravity base structure and the increase of the sectional forces due to increased deformations. Section 3.7.3 similarly shows the effect on vertical panels, and how the equivalent onedimensional elastic buckling length can be calculated for such a two-dimensional structural element. In Section 3.7.4, circular cylindrical shells are reviewed. Finally, Section 3.7.5 shows additional forces on the raft due to large rigid body rotations. 3.7.2 Beam stiffness of shaft
The axial compression on the shaft reduces the bending stiffness and consequently the lateral deformation increases. This is illustrated by an example below. With reference to Fig. 3.6, a free rotation is assumed at the coupling between the shaft and the deck. The deformation function along the shaft can be assumed: w(x) = ␦ · (x) (3.74)
For calculation of buckling load NE, the shape function x is assumed and (3.75) and (3.76) integrated. Alternatively one can use the equations for variable sections from literature. A beam program with linear buckling calculations can also be used. The modal stiffness for a given N will then be from (3.75) and (3.76): K = KM - K K = KM (1-N/NE) With reference to Section 3.4 it is obvious that the natural period for lateral oscillation of the shafts increases by a factor (3.80) The effect on lateral displacement for a given load will be increased by the factor (1 - N / NE)-1 ~1+N / NE (moderate N) (3.81) (3.79)
According to the elastic theory, Equation (3.81) also gives an incremental factor on the curvature and bending moments along the shaft M0 (x)=EI(x) · k0 (x) where ko is the curvature according to the linear solution. The additional moment from second order geometry would then be (3.83) (3.82)
The plane stresses x, y, xy are transformed to principal stresses 1 and 2. Here 2 is the largest compressive stress. The stress 1 will be considered below only if it is compressive. From the geometry and boundary conditions of a planar structural element, the critical buckling stress 2 cr is calculated with regard to the combined stress state 1/2. Tables for ideal plate buckling from (Timoshenko and Gere, 1961) or (Column Research Committee of Japan, 1971) can be used. Another alternative procedure is: a. Deal with each principal stress separately. Calculate the buckling stresses (3.85)
(3.86) where b is the effective plate width which can be different for 1 and 2, t the plate thickness, and D the sectional stiffness. b. Find the relation ␣ between the acting principal stresses, such that
1=␣ · 2(␣ > 0)
(3.87)
c. Critical principal stress 2cr from the combined effect can be derived from the interaction formula (3.88)
d. When the critical stress is found, the equivalent one-dimensional buckling length can be derived from: (3.89) i.e.
moment according to (3.74) above. The two principal directions are to be considered for the incremental moment. 3.7.4 Circular cylindrical shell The same procedure mentioned above in Section 3.7.3 is used to find the second order sectional forces. The only difference is the elastic buckling stresses. Provided the membrane compressive stress 1 is in the hoop direction and 2 in the generatrix direction and that the membrane shear is negligible for second order effects, then the ideal buckling stress for 1 acting alone can be written (Timoshenko and Gere, 1961), or (Column Research Committee of Japan, 1971). (3.91)
where E v t r l = = = = = modulus of elasticity of concrete Poisson’s ratio wall thickness middle radius effective length.
With 2 acting alone, the elastic buckling stress can be written (Timoshenko and Gere, 1961) or (Column Research Committee of Japan, 1971). (3.92) provided that l»1.72 · (rt)0.5 (3.93)
In other words, the vertical stress is half the hoop stress. Practical values can be: Wall thickness Middle radius Cylinder height Modulus of elasticity Poisson’s ratio t = 0.60m r = 12.0m 1 = 30.0m E = 30 000 MPa v = 0.20
From (3.91) the critical buckling stress for the hoop force alone
1E = 112MPa ,(2 = 0)
and for vertical stress alone from (3.92)
(3.96)
2E = 884 MPa , (1 = 0)
(3.97)
From the stress relationship in (3.94) and (3.95) and the buckling stresses in (3.96) and (3.97) it is clear that, generally speaking, the hoop stress is more dominant with regard to second order effects. 3.7.5 Rigid body rotation of a floating structure The computational models for the global effects on the raft, shown in Section 3.5 and Fig. 3.17, are geometrically linear as the rigid body modes are not included, and the deformation modes are assumed to give infinitesimal displacements. Note that displacements include both translation and rotation. This last assumption about the minor displacements from the deformation modes is valid for most of the floating platforms. At the same time, the rigid body rotation in a catenary anchorage situation can be so large that additional forces are exerted on the raft. An uncontrolled accidental water ballasting situation can result in a rigid body rotation (or tilt) of several degrees.
¨ = - g · sin ⌰ X The change in water pressure is ⌬p=p g · X · sin ⌰
(3.98)
(3.99)
For purposes of comparison, a wave acceleration for a 100 year storm of about 1.0 to 1.5 m/s2, leads to the same lateral force on the raft as a rotation of the raft of =6.00. The tilted position, shown in Fig.3.28, can also be critical for the connection between the raft and the deck.
S.P.Timoshenko and W.Krieger (1959): ''Theory of Plates and Shells'', McGraw-Hill. S.P.Timoshenko and J.M.Gere (1961): ''Theory of Elastic Stability'', McGraw-Hill. Column Research Committee of Japan (1971): ''Handbook of Structural Stability'', Corona Publishing Company; Tokyo. T.Sarpkaya and M.Isaacson (1981): ''Mechanics of Wave Forces on Offshore Structures'', van Nostrand Reinhold Company. T.H.Søreide (1981): ''Ultimate Load Analysis of Marine Structures'', Tapir, Norway. References
American Concrete Institute (1995) Building Code Requirements for Structural Concrete (ACI 318–95) and Commentary—ACI 318R 95 (metric versions ACI 318M-95 and ACI 318RM 95) Eurocode 2 European Prestandard ENV 1992–1–1. (1991): Design of concrete structures. CEN 1991 (under revision 1999 for transformation to EN, European Standard). Norwegian Council for Building Standardisation, NBR (1998), Concrete Structures, Design rules. NS 3473, 4th edition, Oslo, Norway, 1992 (in English), 5th edition 1998 (English edition in print).
4 Global analyses
Ivar Holand, SINTEF
4.1 Objective The analyses of offshore structures may be split in several steps: • • • • • load analyses modelling, preprocessing global analysis study of selected areas by non-linear analyses postprocessing, dimensioning, including analysis of reinforcement needed.
distribution of internal section forces in the structure. They also enable the influence of displacements on load effects to be considered. However, when the large number of load cases and the resulting need to superimpose load effects are considered, the linear analysis is likely to remain the primary method in a foreseeable future. Here, a global linear analysis will primarily be discussed, but non-linear analyses will also be touched upon. Dimensioning is discussed in Chapter 5 (Design). In non-linear analyses dimensioning cannot, however, always be separated from analysis. 4.2 Linear finite element methods 4.2.1 Description of methods The structural analyses have, since the mid-seventies, mainly been based on the use of large finite element programs. Description of finite element methods are found in a number of textbooks, e.g. (Bathe, 1982), (Cook et al., 1989), (Zienkiewicz and Taylor, 1989), (NAFEMS, 1991), (Crisfield, 1991), (Saabye Ottoson and Petersson, 1992), (Hinton, 1992). The finite elements may be categorized as: • • • • • • • bar elements beam elements plane stress elements plate bending elements shell elements solid elements several kinds of special elements.
using results known from linear analyses at the boundaries. Such analyses are required in highly stressed intersections between shell type structural members and for slender shell panels, where geometrical non-linearities may also be important. 4.2.2 Program systems A number of program systems are commercially available. It is necessary to choose a program system that has or gives access to relevant pre- and postprocessors, including load generation procedures for the relevant types of loads. Postprocessing is discussed in Chapter 5, Section 5.3.2, where also relevant postprocessors are listed. 4.2.3 Modelling and element meshes for shell surfaces
(b) Cylindrical shell Fig. 4.3 shows a cylindrical wall constituting a part of an axial-symmetric cell with external pressure. The wall is assumed to be clamped at the upper edge. By also clamping the lower edge, symmetry is obtained about a section at the middle of the height, as shown in Fig. 4.3. The model is reasonably close to the actual situation in an external cell in a Condeep platform and is assumed to be representative for a model case suited for an investigation of accuracy. The shell has been modelled by solid elements, and an element model shown in Fig. 4.4. Each element has 8 nodes. Moments and shear forces computed by using the program system NASTRAN are shown in Fig. 4.5. With the simple assumptions a differential equation may also be used for the analysis of the cell. Such an analysis gives a clamping moment of 1.75 MN and a shear force at the edge of 1.873 MN/m. The results from the element analysis (1.71 MN, respectively 1.849 MN/m,) are thus very close to the theoretically exact ones, and the model for the cylinder wall must be considered to be fully satisfactory.
Fig. 4.3 Cylindrical wall. Geometry and edge conditions
4.5 Non-linear analyses 4.5.1 Reasons for non-linear analyses
(a) Objectives When it is deemed necessary to account for the non-linear behaviour of reinforced concrete, particularly because of cracking, additional non-linear analyses are adapted for isolated parts, using results known from linear analyses at the boundaries. Non-linear finite element methods are described e.g. in (Zienkiewicz, O.C. and Taylor, R.L. 1991). In non-linear analyses the superposition principle is not valid, and analyses must be performed for each case separately. A consequence is that such analyses must be restricted to a few critical areas and special load cases. Non-linear methods give significantly more complex numerical analyses than the linear ones. Hence, much more care must be exercised to avoid malfunction of such programs; for instance that the analyses are aborted too early by a convergence criterion that does not function as planned. Reasons why linear analyses are not sufficient in some case may be divided into two categories: geometric non-linearities and material non-linearities. (b) Geometric non-linearity Geometric non-linearity occurs when the structure is so slender that displacements play a significant role for the static behaviour. Typical examples are the curved walls in cells and shafts in Condeep platforms. In these walls, displacements combined with deviations from ideal geometry and large axial forces in particular in the arch direction cause additional moments in the walls that are of a magnitude which must be considered in the design (risk of implosion). For the bending of a shaft as a long column, the axial loads may also give significant additional moments because of lateral deflections. (c) Material non-linearities Material non-linearity occurs in all design of reinforced concrete, for example when cracking of concrete and yielding of reinforcement is considered. Here, however, only the cases are considered where this causes significant changes in the load effects. Relevant locations are found where cell walls meet, in the transition from the vertical wall to the dome etc. Non-linear analyses in such cases can contribute to giving improved understanding of the real behaviour of reinforced concrete, and may be applied directly in design. In such analyses, the need to consider multi-axial stresses and strains will often arise. Such considerations are generally outside the rules in codes and standards. Hence, a description is needed of the material models which the actual program applies, and the models must be assessed in relation to the requirements in codes and standards, in addition to research results and experience that are not implemented in rules. Related topics are discussed in Chapter 5. 4.5.2 Fracture mechanics
intruding in a crack as the crack progresses, a case which may occur where external curved walls in Condeep platforms meet, where there is risk of delamination (see Chapter 5), etc. Fracture mechanics considerations are based on the recording of fracture energy in small specimens, and should be applied with care until the results from analyses have been calibrated with larger-scale tests. 4.5.3 Program systems A number of program systems for non-linear analyses are available on the market. The following program systems have been applied for non-linear analyses of concrete platforms: • • • • Fenris (Det Norske Veritas Sesam AS, Høvik, Norway) Abaqus (Hibbitt, Karlsson & Sorensen, Pawtucket, RI, USA) Diana (Diana Analysis, Delft, The Netherlands) Solvia (Solvia Engineering AB, Linköping, Sweden).
example of guidelines for independent calculations the following guidelines by the Norwegian Petroleum Directorate (NPD, 1992) are quoted: “The design of structures or structural parts of significance to the overall safety should be verified by means of independent calculations. Such verification may be carried out by manual calculations or by computer calculations. When computer calculations are used, it is assumed that the person carrying out the verification uses another software programme than the designer. Software used in verifications should itself be verified for the purpose in question. The necessary calculations should be sufficiently accurate and extensive to clearly demonstrate that the dimensions are adequate”. 4.6.2 Checklists
A checklist for verification of element analyses must be adapted to the verification procedures proposed in Chapter 7. Relevant key words are: • • • • • • • • • • • • • qualification requirements linear/non-linear analyses program system element type element mesh input data accuracy of element model (checking report) load vectors transfer between pre-processor and main analysis, storing transfer of data to postprocessor retrieval of correct data (make certain that the last corrected data are used) transfer of analysis results to design (often: stresses to stress resultants) numerical accuracy
Special items for numerical accuracy are: • • • • • • precision level tolerances (frequently user controlled) condition numbers residual forces checking capabilities in the program what is default and what can be controlled by the user?
Special key words for non-linear analyses are: • • • • material models for concrete and reinforcement cracking criteria strain assumptions (for example plane stress, plane strain) modelling of reinforcement bars (for example individual bars or smeared)
connection of individual bars to nodes water pressure in cracks geometric deviations convergence criteria fracture mechanics analysis Qualification requirements
Norwegian Petroleum Directorate (1992) Regulations concerning loadbearing structures in the petroleum activities, stipulated by the Norwegian Petroleum Directorate, Stavanger, Norway. Mathisen, K.M., Kvamsdal, T. and Okstad, K.M. (1994) Techniques for Reliable Calculation of Sectional Forces in Concrete Structures Based on Finite Element Computations. Department of Structural Engineering, The Norwegian Institute of Technology. Kvamsdal, T. and Mathisen, K.M. (1994) Reliable Recovery of Stress Resultants. First Diana Conference on Computational Mechanics, Delft, The Netherlands. Saabye Ottosen, N. and Petersson, H. (1992) Introduction to the Finite Element Method. Prentice Hall, 1992. Zienkiewicz, O.C. and Taylor, R.L. (1989) The Finite Element Method, 4th Ed, Vol. 1 Basic Formulation and Linear Problems, McGraw-Hill, London. Zienkiewicz, O.C. and Taylor, R.L. (1991) The Finite Element Method, 4th Ed, Vol. 2: Solid and Fluid Mechanics, Dynamics and Non-Linearity. McGraw Hill, London.
5 Design
Erik Thorenfeldt, SINTEF
5.1 Typical structures and structural parts Typical offshore concrete structures are discussed in Chapter 1, see Figs. 1.1, 1.2, 1.3 and 1.4. A sketch of a typical Condeep structure is shown in Fig. 2.1 and a sketch of a tension leg platform in Fig. 2.2. Typical structural parts and loadings appear for Condeeps from Figs. 3.4, 3.5 and 3.11, and for a floater from Figs. 3.12 and 3.13. The structures are mainly cell structures, which in principle are composed of slabs, plates and shell elements. Simple massive beams, columns and frames occur relatively seldomly. Columns and frames as parts of the main structure usually have cross sections in the form of hollow cylinders or rectangular boxes which are designed locally as slab/plates/shell elements. A Condeep structure is usually divided into skirts, lower domes, cell walls, upper domes and shafts (see Fig. 2.1). A floating platform will comprise other typical structural parts: pontoons, cylindrical columns and box beams. Design of offshore concrete structures is in many respects similar to the design of large structures in general. The typical characteristics are the complexity caused by the numerous disciplines involved, among them • • • • • • soil mechanics loads from wind, waves and current accidental actions dynamic forces dynamic structural response non-linearities.
This Chapter 5 mainly discusses the design of the concrete structure itself when the analyses have been completed and the load effects determined. The main emphasis is placed on typical aspects which are especially important for producing a safe design. In all types of platform structures the intersections between the different shell and plate structures represent critical parts of the structure and the design. Some important intersections between the different structural parts of a Condeep platform are shown in Fig. 5.1.
Fig. 5.1 Typical intersection regions (nodes) in a cell structure (Gullfaks-C) 1. 2. 3. 4. Lower ring beam between the cell wall, lower dome and skirt Upper ring beam between the cell wall, upper dome (and shaft) Intersection between outer caisson cells Intersection between joint cell wall and tri-cell walls
In addition to the general design basis it will be practical to prepare documents for each activity in the design, which thoroughly discuss the application of the design basis and gather supplementary information. The activities within a large project may typically be: load analysis, dynamic analysis, static analysis, post processing, design of main structural parts, design of special parts for attachment of mechanical equipment, temporary structures for use in the construction phase, etc. These documents are named Design Briefs. The Design Basis and Design Briefs are management documents for all design work to be performed within the project and its part activities. To obtain safe accomplishment of the project it is required that these documents be worked out and presented to the client for approval at an early stage. 5.2.2 Design basis (a) Topics in Design Basis The Design Basis will usually address the following topics: • • • • • • • • • • • • The client’s most important functional requirements Reference to rules, regulations, standards and specifications Possible deviations from standards Design principles and limit states Temporary and permanent construction phases Loads, load combinations and load factors Material coefficients Materials and material parameters General reinforcement detailing Design assumptions and criteria Design procedures and methods Interface areas.
If, at some point, the rules put forward in the Design Basis deviate from the rules in the references, this should be clearly stated. All documents used in the preparation of the Design Basis should be listed in a reference list. (c) The client’s functional requirements The client’s functional requirements are usually specified in a separate document, where the conditions for management and use of the structure are described. In addition to a reference to this document it would be practical to quote for example: • • • • • Maximum and minimum deck weight with centre of gravity Environmental loads Lifetime of the structure Water depths at the field The orientation of the structure.
(d) Design principles The structures are usually designed according to the partial safety factor method. Under certain conditions the safety of the structure may also be assessed on the basis of probabilistic methods with specified safety indices. This approach is mainly used in connection with special accidental loadings. In special cases a testing of structural parts may also be applied. It should be explicitly stated in the Design Basis document if such methods are to be applied. The structures are usually designed in the following limit states: • • • • Ultimate limit state Serviceability limit state Fatigue limit state Accidental limit state (progressive collapse) ULS SLS FLS PLS.
Different sub-phases during construction will often be decisive for the design of traditional platform structures. These include floating out of dock or different floating phases during further construction and especially the almost complete submersion of the structure for deck-mating. (f) Loads, load combinations and load factors The loads acting on an offshore concrete structure have different characteristics. As an example, the Norwegian Petroleum Directorate (NPD, 1992) categorizes loads as permanent loads (P), live loads/variable functional loads (L), environmental loads (E), deformation loads (D), and accidental loads (A). Loads with categorization and detailed specifications concerning establishment of characteristic values will be provided by the client. As an example, see (Statoil, 1992). Usually, permanent loads from self-weight and water pressure combined with environmental loads due to waves and wind will have a dominating influence on the design of the concrete structure. Determination of the characteristic environmental loads are based on observations at the site and the calculation of wind and sea states, according to (NPD, 1992) with 100 years return period. For serviceability limit state criteria or for temporary states shorter return periods are used, such as 1 year. In order to determine the static equivalent design load on the basis of dynamic environmental loading, separate analyses are performed which take account of the stochastic variation of the loads and response of the structure. The designing wave load may therefore take different values for different parts of the structure. Table 5.1 Example of load factors and combinations of loads (NPD, 1992)
General requirements concerning loads, and especially which types of accidental loads are to be considered (ship collisions, falling objects, explosions, fire, earthquake, loss of internal pressure, erroneous trimming of the ballast, etc.) are to be given in the Design Basis. Furthermore, the extent to which analysis of the consequences of local damage is required, with possible flooding of parts of the cell structure, must also be specified. Design load combinations and load factors should be based on analysis of the uncertainty of the load effects from combinations of typical loads acting on offshore structures, and will be specified in national or international regulations, or by the client. An example is found in Table 5.1. (g) Material safety factors Material safety factors will also be specified in national or international regulations. As an example, material factors used for offshore structures in Norway are given in Table 5.2. Table 5.2 Example of material factors to be applied in design (NPD, 1992)
(i) Reinforcement principles The Design Basis document should outline the basic principles for the reinforcement system in order to ensure a unified detailing of the reinforcement in the whole structure. This will typically comprise the required minimum reinforcement, maximum spacing, standard bending diameters, methods and standard dimensions of splices and anchorages, limitations of maximum reinforcement density; use of bundled bars, etc. The prestressing system with standard cable dimensions will also be specified in the Design Basis. (j) Design assumptions and criteria The main parts of a concrete platform are classified in a high safety class taking into account that a failure situation may result in catastrophic consequences with high risk for loss of human lives. The extent of control measures is to be evaluated especially. Regarding control of the design and construction, reference is made to Chapters 6 and 7. Examples of relevant specifications are: For the serviceability limit state: • design exposure class (with corresponding concrete cover and crack widths) • structural requirements to ensure strict water tightness • criteria concerning vibrations and displacements, especially for shaft structures. For the fatigue limit state: load distribution spectra and lifetime factors. For dimension tolerances: • • • • • thickness of each structural part deviations from the intended centre line of the components concrete cover position of the reinforcement. deviations from the ideal middle plane of the structure (as a basis for the design of slender shell structures).
• Additional design tasks which are not naturally included in the general design of the structure, like embedded steel structures, crane support structures, tube penetrations, temporary block-outs, etc. • interface areas to other main parts of the concrete structure and to other design disciplines requiring procedures regarding management of interface information and co-ordination. Practical experience has shown that it is a considerable challenge to handle all the information necessary to satisfy all the different requirements in such interface areas, and that this is a source of possible design errors. An example of the last item is that an offshore oil production platform will be equipped with a large number of tubes which are originally planned and designed by designers in other disciplines. The connection of such tube systems to the concrete structure will often require specific limitations of the load effects (deformations) in certain regions of the concrete structure. The technical solution to such problems should be worked out in close co-operation between process equipment and concrete structure designers to ensure that the combined structure performs as planned and the integrity of the concrete structure is taken care of. 5.2.3 Design briefs (a) Needs for design briefs In addition to the Design Basis document, a series of sub-documents in the form of design briefs for each main activity, such as load analysis, structural FEM analysis and detail design of each main part of the structure should be worked out as part of the Design Briefs. The basis for the Design Brief for a part of the concrete structure will be: • • • • relevant results obtained in the concept development phase drawings of the geometry of the part relevant parts of the Design Basis explaining how the loads are carried by the structure construction phases and limit states the structural part is to be designed for.
The Design Brief will be an extension and further detailing of the general topics in the Design Basis. The outline of the Design Briefs should be similar, but the topics to be discussed may vary depending on the type of activity or type of structure. The Design Brief will also include a description of how the design work is to be performed. It is often experienced that questions of a character where clarification with the client is necessary arise during the preparation of the Design Brief. The document will therefore also be helpful in clarification of all important questions in due time before starting the work-intensive production of design documents and drawings. The following list of typical topics to be discussed in the design brief is not complete and should be supplemented in each case: • • • • Define documents and drawings to be produced within the activity Depict and describe the configuration of the structural part Describe interfaces to other parts and disciplines Indicate key data of load effects and geometry
Describe the structural system Describe relevant construction phases and load combinations Indicate all construction phases and limit states to be checked Describe the load effects included in the global element analyses Describe analyses methods for load effects not included in the global analyses Describe design methods and design sections to be applied Reinforcement system and standard detailing, prestressing system and configuration Criticality and sensitivity assessment Indicate the relevant post-processing files Discuss the implementation of the quality assurance system for the particular task.
to provide the detail designers with a general view on the design task and help them in understanding the structural behaviour. (e) Construction phases and load combinations The design loads are established by scaling the unit load cases used in the analysis by multiplication with scaling factors for the characteristic value and load factors according to the actual limit state and load combination. Each load combination consists of a number of unit load cases which are scaled and added by linear superposition. Equilibrium Load Cases consist of a set of active loads (self weight, self weight and functional loads on deck structure, weight of equipment, weight of ballasting, water pressure and environmental loads) and reaction loads (reactive soil pressure on ground based structures in operational phase or buoyancy forces and reaction forces in tethers in floating phases). In the operational phase the structure will be designed for possible variation of the deck weight, variation of the effect of prestressing and the effect of waves and wind in various directions. Furthermore, the possibilities of different possible distributions of the soil reactions are usually accounted for. Assume that a design section in the structure is to be designed in Ultimate Limit State for: • • • • • 12 different wave directions 2 deck weights (max and min) 2 soil pressure distributions 2 effects of prestressing (max and min) 2 load combinations (a and b in ultimate limit state).
The example results in 192 load cases which are to be checked. The number of load cases increases considerably in dynamic and fatigue response calculations. As mentioned in Section 5.2.2, the lifetime of the structure consists of several phases. The total number of phases will mainly depend on the number of sub-phases during construction. The transportation, installation, operation and removal phases will exist for all typical offshore structures. Due to the large number of load combinations and construction phases it is practical to assess which combinations/phases will certainly not be decisive in the design and can therefore be excluded. This evaluation is commonly based on simplified calculation methods of the same type as also used in the verification of the design (see Chapters 3 and 7). The results of these calculations are included in a separate load-phase document. An abstract of this document explaining for which construction phases design calculations are to be made and which load combinations and limit states are to be checked in each phase shall be included in the Design Brief. The principles for establishment of the design load combinations should also be included. However, the detailed combination of unit load cases used in the analysis will be described in separate referenced documents. All special load combinations which are not included in the global analysis must be described and explained in detail, to avoid time-consuming discussions at a later stage. Typical examples are combinations with local loads such as ship collisions and impact from falling objects, local implosion or other non-linear effects.
changes must be available. The revision log must include the dates of implementation, the cause of the changes and the person responsible. The revision list of each file may conveniently be included in the heading. 5.3 Design procedures 5.3.1 Design process
In Sections 5.4 and 5.5 the design activities, i.e. the design and detailing of the main concrete structural parts are discussed. For further discussion of the analysis, reference is made to Chapters 3 and 4. All personnel performing technical tasks must know the content of the Design Basis and understand the application of this document in their own work. Furthermore, the personnel must be familiar with the content and application of the Design Brief for the actual part of structure where they are involved in the design. Fig. 5.2 shows a simplified flow chart of the design process based on the use of a postprocessor. The design of concrete sections is based on integrated load effects from the finite element analysis; see also Chapter 3. Due to the large number of load cases and sections to be checked, a high degree of automation of the design process is necessary. As far as possible a postprocessor is used as a design tool. It is important to be aware of the inherent limitations of the validity of a section-by-section design based on forces from linear elastic analysis. The linear analysis cannot simulate all important load effects with satisfactory accuracy. Local analysis and manual design must therefore supplement the analysis. The results of the supplementary design are added to or substitute the results from the postprocessor. An automatic design process represents a demanding challenge to the designer regarding a thorough assessment of the results. Independent simplified calculations must be done to verify that the calculated amounts of reinforcement and utilization of the concrete sections are reasonable. See also Chapter 3 (Simplified analysis) and Chapter 6 (Verification). 5.3.2 Postprocessor
The number and position of the integration points (Gauss-points), where the stresses may be integrated by the postprocessor, is determined by the choice of the finite element model and the type of elements. The choice of the element mesh for the analysis also decides the possible locations of the design points. Good communication between the analysis team and the team responsible for the postprocessing is required to ensure that the practical use of the analysis results in the detail design is considered in the element mesh development.
extrapolated from the results at integration points further into the regular part of the structure. Extrapolation methods are discussed in Chapter 3. Acceptably small analysis errors in regular design points are easily unacceptably enhanced by extrapolation. The accuracy must be closely validated. All regions of the structure where the analysis is expected to give an inaccurate result should be documented in a separate report. This report should be used actively in the design process to ensure that supplementary analysis of these regions is carried out. In regions with varying shell thickness it is especially important to choose the position and direction of the design section in order to obtain representative section forces. Sections normal to the middle plane of the elements are usually preferable (see Chapter 3).
relatively high risk that human errors will occur. This again emphasizes the importance of a responsible and critical use of the automatic design tool. The results should be critically assessed and not be regarded as an authorized answer to correct design. All software to be deployed in detailed design of marine concrete structures is to be verified for this application (NPD, 1992). The ISO standard (ISO13819 Part 3) will, when available, for design refer to NS 3473 as a standard that covers relevant conditions (see Chapter 1, Section 1.6.2). It will still be advantageous for further international application if the postprocessor also has the possibility of performing the design checking according to other standards, e.g. the British Standard (BS), Canadian Standard (Can Standard) or the American ACI codes, and Eurocode 2. Design according to other codes is already implemented in some of the existing postprocessors. In any case, the postprocessor program should be prepared for easy implementation of optional design codes. 5.3.3 Effect of water pressure in cracked concrete
According to the simplified method in NS3473 a compressive axial force will increase the shear capacity considerably. It is recommended that the axial force calculated by the global analysis with water pressure at the external surface is conservatively reduced by a force resultant corresponding to full water pressure in the section area. The upper limit of the shear capacity formula will often be decisive for structural members with high compressive axial force. When high compressive stresses exist in the principal shear direction of a slab or shell or, maybe more often, normal to this direction, the tensile strength in the third principal direction will decrease. A full utilization of the shear strength according to the simplified method for members without shear reinforcement may in such cases turn out to be non-conservative. Application of the alternative truss model method for shear design will result in a general need for shear reinforcement in all members subjected to transverse shear. This seems unnecessaryly conservative for slabs and shells in general. Methods based on the modified compression field theory (MCFT) (Collins and Michell, 1991) represent a further option which is also introduced in the so-called general method in NS3473. This method is primarily intended for members with in-plane shear, but is also applicable for beams and slabs with shear reinforcement. Based on the notion of the mean tensile strength of cracked (reinforced) concrete and the ability of shear transfer in cracks, the method also features a “concrete contribution” to the shear strength, but the method is apparently not sufficiently well verified to be applied in the shear design of members without shear reinforcement. The need for minimum shear reinforcement in platform structures is further discussed in Section 5.4.3.
However, when water pressure penetrates into cracks in areas with discontinuous geometry, such as the sharp inner corner of the outer intersections between two main cells in the caisson, or the internal corner of a tri-cell between the main cells in a Condeep structure, an extra transverse load in the form of a wedge-effect will occur. This load will add to the load on the exposed surface taken into account in the analysis. This effect is illustrated in Fig. 5.8.
• The effective cross section of the compression struts or compression fields is to be assumed in accordance with recognized calculation models. The design compressive strength of the concrete in the struts is to take into account the effect of the cracking parallel to the principal compressive strain direction. A modified design strength dependent on the transverse strain is given, e.g. in NS3473. This implies that the transverse strains must be controlled and limited. Usually a simplified assessment of the transverse strain as a basis for the design of the distributed reinforcement directed approximately normal to the struts is required. • The transverse reinforcement must be able to resist tensile forces due to possible deviations from the assumed compression field. Insufficient transverse reinforcement may lead to splitting of the compression struts with a shear failure through the compression field as a possible consequence. • The compression capacity may be decisive in concentrated joints. Simplified rules, especially applicable for concentrated loads and supports are given, e.g. in NS 3473. In joints where one or more tensile ties will be anchored, it is important that the reinforcement is safely anchored in, or behind, the joint area to ensure a safe transfer of forces between the tensile ties and the compression struts. A main challenge as regards the use of simplified force models is to develop models in sufficient conformity with general strain compatibility requirements. If there is no recognized calculation model for the member and states of stress in question, the geometry of the model may be developed on the basis of load testing or theoretical analysis based on strain compatibility of members with similar or comparable geometry. Analysis of the member by linear finite element methods may give valuable support for the choice of force directions and force distribution in statically indeterminate models. Linear analysis will, however, not give realistic results in cases where the cracking of the concrete will significantly change the flow of forces in the area, e.g. in members with tensile stresses around sharp internal corners. Calculation models for typical discontinuity regions may be verified and modified by nonlinear finite element analyses where the reinforcement units and the cracking of the concrete are modelled. It is, however, important to be aware of the limitation of the validity of current practically applicable non-linear analysis methods where, for example, an unrealistic modelling of perfect bond between concrete and steel is usually applied. For important, typical intersection areas, load testing of the prototype or scaled down physical specimens may be required. According to NS3473, force models should be applied in order to determine internal forces at distances less than the effective depth (d) from the support or from concentrated loads. The provision is especially intended to ensure safe transfer of forces to concentrated supports. In shell structure intersection regions with more gradual transition into corner areas, often with indirect support of one structural member on the other, it is important to define the sections where the shear capacity verification should change from the simplified method to a force model (truss, strut and tie or compression field model). An example is shown in Fig. 5.9.
A pure bending moment is transferred more naturally by the simple reinforcement with almost constant internal lever arm around the 60-degree corner shown in Fig. 5.10 c). The tensile force resultant of the inside reinforcements is naturally intersecting the deviation angles of the compression struts. However, to avoid excessive width of the tensile crack occurring in the inner corner, a transverse reinforcement crossing the inner corner as shown in Fig. 5.10 d) is recommended. Fig. 5.12 shows corners with significant shear forces and corresponding balancing axial forces in addition to the opening bending moments. In the 90-degree corner in Fig. 5.11 a) the shear force in one of the adjacent walls may be picked up by the main tensile reinforcement in the opposite wall. Supplementary reinforcement is not necessary, however, the anchorage of the tensile reinforcement close to the outside compression face is increasingly important. The shear force in the 60-degree corner in Fig. 5.11 b) cannot be transferred directly to the main reinforcement in the opposite wall. This reinforcement is slanting 30 degrees to the “wrong” side compared to the principal tensile direction of the stresses at the axis of the adjacent wall. Safe transfer of the shear forces depends entirely on the transverse reinforcement A suspension support of the walls is established by the transverse corner reinforcement. The support function of the reinforcement is enhanced if the corner is equipped with a haunch with the reinforcement placed in the haunch as shown in Fig. 5.11 b). In this case the reinforcement will also take over the transfer of the tensile force due to the opening corner moment. The transverse reinforcement crossing the inner corner is also recommended for 90-degree corners to minimize the width of the corner crack. The intersections between the tri-cell walls and the joint main-cell wall in a Condeep structure with high water pressure externally and inside the tri-cells will be subjected to large axial compression forces in the walls in addition to moment and shear in the tri-cell corners. The axial forces are often dominating, i.e. the components normal to the joint wall of the axial forces and the shear forces in the tri-cell walls result in transverse compression in the theoretical node point of the intersection. This is often the case also when additional wedge forces due to water pressure in cracks are taken into account. Large axial forces compared to the opening moment will also tend to minimize the tension forces in the reinforcement at the inside of the tri-cell walls near the inner corner. Due to the thickness of the walls, the inside faces will, however, meet at the inner corner at a considerable distance from the theoretical intersection point of the wall centre-lines. The ratio of the thickness of the tri-cell wall to the joint wall will increase this distance. Due to the strain gradient in the corner, large transversal tensile stresses may occur near the inner corner in spite of the large axial compression forces. The transverse stress resultant will increase radically by the introduction of a corner haunch. In the simplified force model of the Sleipner A1 tri-cell corner shown in Fig. 5.12 a), the component normal to the tri-cell wall of the tensile force (Fsv) in the transverse reinforcement in the haunch must equilibrate the full shear force of the tri-cell wall. This reinforcement must also transfer the resulting tensile forces due to opening moment/axial force and additional wedge forces due to water pressure in cracks in the haunch. Supplementary distributed transverse reinforcement is necessary to balance water pressure on cracks deeper into the intersection region.
Fig. 5.11 Corners with opening moment, shear force and axial force The problems of establishing a consistent model when the transverse reinforcement is too short are indicated in Fig. 5.12 b). Especially region A, with little or no links between the wall faces, will be over-stressed. The tensile crack occurring behind the anchor plate of the Theaded bar will initiate the development of a shear failure through the corner area.
It is important to calculate the tensile forces due to the deviation of the general forces around anchor zones and where the prestressing cables themselves change direction, and provide the necessary reinforcement. Walls and shells are to be equipped with both transversal and extra longitudinal reinforcement in the anchorage zones. 5.4 Reinforcement 5.4.1 Basis for reinforcement design All reinforcement is to comply with the requirements in relevant codes and the corresponding product standards. The required reinforcement amount calculated by the postprocessor and supplementary manual calculations is the basis for the preparation of reinforcement drawings and schedules. The reinforcement systems used for offshore structures are principally the same as for onshore structures. The main practical differences from ordinary structures are the large dimensions and high loads, requiring particularly heavy reinforcement. Furthermore, large parts of concrete platforms are slip-formed, e.g. skirts, cell walls and shafts. The comments concerning practical detailing and reinforcement systems in Section 5.4.3 are therefore mainly related to slip-form construction. The reinforcement production documents comprise reinforcement schedules, special listings of reinforcement for slip-form construction, “reinforcement keys” etc, in addition to ordinary reinforcement drawings. 5.4.2 Minimum surface reinforcement
A reduced percentage of surface reinforcement may still give satisfactory crack distribution in thick structures. The main reason is that the tension forces transferred by reinforcement bonds introduce unevenly distributed tensile stresses in the concrete section. The required minimum reinforcement is increased in areas of the structure exposed to high water pressure. The reinforcement must resist the additional resultant of the water pressure at the crack surface. The minimum reinforcement in walls and shells according to the NPD-guidelines (NPD, 1992) takes into account the thickness of the structure and the water pressure. The required minimum amount at each face and in each main direction is as follows: As min=k Ac (ftk+av)ftk/fsk where k is a factor varying from 0.4 for wall thickness 300 mm to 0.25 for wall thickness > 800 mm. (interpolation for intermediate wall thicknesses) v is the actual water pressure ␣ is a factor, which is taken as 1.0 if the relevance of lower values is not documented Ac is the area of the concrete section ftk is the tensile strength of the concrete fsk is the yield strength of the reinforcement.
The theoretical k-value is 0.5 if the stress at cracking is constant through the thickness and ftk is the actual tensile strength. Crack distribution is therefore not guaranteed by the required minimum reinforcement if this situation can occur. Prestressed reinforcement in injected ducts only contributes slightly to the distribution of cracks. The general requirement regarding compressive reinforcement is not so clear. Standard requirements have been related to general assessment of the safety of reinforced concrete structures. The increased safety of sections consisting of two materials with uncorrelated strength variables may be regarded as a condition for the use of material safety coefficients for reinforced concrete. The risk that tension may occur in unexpected directions, e.g. due to accidental loads, is a further argument for requiring a minimum amount of reinforcement. The direction of the internal forces in shells and wall members of offshore structures subjected to environmental loads may vary considerably. Equal minimum reinforcement at both faces and in both main directions is therefore usually required. 5.4.3 Minimum transverse reinforcement
500 MPa this corresponds to, for example, Ø12 mm reinforcement links with spacing of 270 mm in both main directions. There are several special reasons for requiring a general minimum transversal reinforcement in walls and shells in large marine structures. Some of these reasons are: • • • • • Scale factors for thick structures The use of concrete with high strength and brittleness Water pressure in pores and cracks Frequently lap sliced reinforcement High bi-axial compression stresses.
reinforcement is to be shaped in accordance with the analytical model and anchored safely at the assumed local joints. The general requirements concerning the local positioning of the reinforcement is stated in NS3473 as follows: • Reinforcement shall be placed in such a way that concreting will not be obstructed and so that sufficient bond anchorage, corrosion protection and fire resistance will be achieved. • The positions of ribbed bars may be designed in accordance with the given minimum spacings without regard to the ribs, but the actual outer dimensions shall be taken into account when calculating clearance for placing of reinforcement and execution of the concreting. • The positioning of the reinforcement shall be designed so that the given requirements to the concrete cover can be obtained in compliance with the specified tolerances. The minimum theoretical clearance between single bars or bundles according to (NS 3473, 1992) is 2 Øe (or 1.5 Øe in lap splicing areas), where Øe is the equivalent diameter of bundles. Bundles are widely used in large structures, but more than 2 bars in each bundle (3 bars at the splices) is avoided as far as possible. It is important to take into account the actual outer dimensions of ribbed bars when reinforcement in several layers is used. Production of realistic detail drawings (and sometimes prototype tests) of intersection regions is necessary to avoid obstructions during the construction work; (see Fig. 5.14). Fig. 5.15 shows a simplified sketch of the reinforcement in a slipformed cell wall. The reinforcement must be directed vertically and horizontally due to the yokes carrying the formwork and due to the continuous lifting operation. The horizontal “hoop” reinforcement must be inserted beneath the yokes. The hoop reinforcement is preferably placed in the outer layer outside the vertical reinforcement. The placing of more than one reinforcement layer on each side of the curved wall is difficult. The second horizontal layer must in this case be placed inside the inner vertical layer, but handling of the curved bars in the internal of the wall is still difficult, and the practical bar lengths will be very limited. Reinforcement in several layers is easier to accomplish in plane walls with straight bars. The horizontal reinforcement is indicated on the drawings with the required constant spacing of the bundles. The required vertical reinforcement intensity is to be recalculated and indicated by the equivalent number of bundles between the yokes. The diameter of the shaft structures will typically vary with the height levels. The distance between the yokes and the number of reinforcement bundles will vary accordingly. The vertical reinforcement is placed inside guidance racks attached to the slipform construction. The vertical position and a reasonable distribution of the bundles are thereby secured. Vertical bars are placed as close as possible to the yokes to minimize the inevitable increased spacing due to the width of the yokes (typically 200–250 mm). The designer must take into account the necessary practical adjustment of the reinforcement in slipform construction by the choice of a reasonably large theoretical spacing in order to avoid violation of the minimum spacing requirement in practice. The spacing of the vertical bars can be adjusted (if needed) where the hoop reinforcement bars are tied to the vertical reinforcement.
documents. This concerns primarily the personnel responsible for the reinforcement, but the personnel responsible for the concreting should also be consulted, especially when the density of the reinforcement will require special workability of the concrete. In that case it may also be necessary to determine the position of the concreting tubes in advance. Thorough discussions with the responsible personnel at the site and adjustment of the reinforcement in order to simplify the execution, but without violation of the required continuity and safe anchorage of the reinforcement, will certainly result in more optimal reinforcement solutions. The staff on site has generally the best knowledge of the problems occurring during the execution of the reinforcement. Some of the points of importance regarding the mounting of the reinforcement are: • Practical lengths and shapes of the reinforcement (handling weight limits, simplicity of production and installation) • The number of regions with different reinforcement (a large number of variants will generally increase the risk of errors and delay) • Practical and clear production documents (drawings, lists, etc). It is often necessary to increase the number of workers drastically within the limited time periods when the large main parts of the structures are reinforced and concreted. This is especially the case when executing large slipform constructions on continuous shift-work basis. In addition to the proper training of all personnel on site, the best guarantee for a successful result is the choice of a simple reinforcement without unnecessary large number of reinforcement variants. Reinforcement shapes or amounts are not to be changed on site without the explicit approval by the responsible designer. All changes are to be reported, registered and evaluated. The changes are finally summarized in updated drawings showing the final structure as built. References Bjerkeli, L.M. (1990) Water Pressure on Concrete Structures. Dr.ing Thesis 1990:31, Division of Concrete Structures, The Norwegian Institute of Technology. Brekke, D.-E., Åldstedt, E. and Grosch, H. (1994) Design of Offshore Concrete Structures Based on Postprocessing of Results from Finite Element Analysis (FEA): Methods, Limitations and Accuracy. Proceedings of the Fourth International Offshore and Polar Conference, ISOPE, Osaka, Japan, pp. 318–28. CEB-FIP. Model Code 1990 (1993). Thomas Telford Services, London CEN (Comité Européen du Normalisation) (1991) European Prestandard ENV 1992–1– 1.Eurocode 2. Design of concrete structures . Collins, M.P. and Mitchell, D. (1991) Prestressed Concrete Structures. Prentice-Hall.
ISO standard 13819 Part 3 (to appear, will cover the entire engineering process for offshore concrete structures). Jakobsen, B., Gausel, E., Stemland, H. and Tomaszewicz, A., (1993) Large-scale tests on cell wall joints of a concrete base structure. High Strength Concrete 1993 Proceedings. Lillehammer, Norway. Norwegian Council for Building Standardisation, NBR (1998), Norwegian Standard NS 3473 Concrete Structures, Design rules, 4th edition, Oslo, Norway, 1992 (in English), 5th edition 1998 (English edition in print). Norwegian Council for Building Standardisation (NBR) (1990) Norwegian Standard NS 3479 Design of Structures. Design Loads. 3rd Edition (In Norwegian). Norwegian Petroleum Directorate (NPD) (1992) Regulations Concerning Loadbearing Structures in the Petroleum Activities. Including guidelines for structural design of concrete structures, stipulated by NPD, Stavanger, Norway. Statoil (1992) N-SD-001. Specification for Design. Structural Design for Offshore Installations.
6 Quality assurance
Erik Jersin, SINTEF
6.1 Introduction 6.1.1 Purpose and limitations of Chapter 6 The purpose of this chapter is to give a general view of the basic demands for Quality Assurance and throw light on what these imply when it comes to engineering, design and dimensioning of concrete structures at sea. The description particularly concentrates on important elements, considering the scope of this book. Consequently, this chapter does not give a complete picture of which requirements should be fulfilled, for instance to fully satisfy ISO 9001:1994 Quality Systems. Model for quality assurance in design, development, production, installation and servicing. Thus, among others the following important elements in ISO 9001:1994 have only been dealt with superficially: 4.1 Management responsibility 4.5 Document and data control 4.13 Control of non-conforming products 4.14 Corrective and preventive action 4.16 Control of quality records In addition, Quality Assurance related to the development and use of software has not been dealt with here. Particular standards for the above-mentioned exist, e.g. ISO 9000–3:1992 Quality Management and Quality Assurance standards, Part 3: Guidelines for the application of ISO 9001 in development, supply and maintenance of software. 6.1.2 Special features of engineering and design of concrete structures at sea
activities on the Norwegian Continental Shelf with comments (“The Internal Control Regulation”) In addition, several guides are in frequent use. These are not regulatory documents, but offer guidelines. Particular attention should be drawn to the following: • ISO 9004–1:1994 Quality management and quality system elements. Part 1: Guidelines. • ISO 9000–3:1992 Quality management and quality assurance standards. Part 3: Guidelines for the application of ISO 9001 to the development, supply and maintenance of software. • ISO 10011:1992 Guidelines for auditing quality systems. Parts 1, 2 and 3. • ISO 9004–4:1995 Quality management and quality system elements. Part 4: Guidelines for quality improvement. Note that “The ISO 9000-family” is being revised at present. The target dates for publishing the revised standards are year 2000–2001. However, there is no reason to assume that the actual quality assurance requirements in the revised standards will be significantly changed. 6.2.2 The three basic principles of Quality Assurance
In view of the large number of documents that exist when it comes to Quality Management (QM) and Quality Assurance (QA), it is important to keep in mind the following three simple and basic QA principles: • Prevent • Detect • Correct These principles in fact form the three-pronged basic strategy of all commonly used QA standards and guidelines to day. Table 6.1 gives a broad outline of how the most usual Quality Assurance requirements can be grouped accordingly. Each element is described in more detail below. Table 6.1. The basic principles of Quality Assurance and some corresponding requirements
occurred, i.e. to find the root cause, and implement the necessary measures to prevent recurrences. Such measures are usually called Corrective Actions. The analysis also includes an evaluation of the present nonconformity in view of previous ones, in order to discover and correct prospective unfortunate trends as early as possible. If, for example, a significantly increasing number of defects have been detected on drawings after they have been distributed, this could be a reason for tightening up the Self-Check requirement on the designers. Alternatively, the drawing software programs should be re-checked (verified). 6.3 Quality assurance in engineering and design of concrete structures 6.3.1 The ISO requirements for Quality Assurance
ISO 9001 describes a series of general requirements to the quality system in engineering and design, construction, installation and servicing. In this chapter we presuppose that such a basic system has been implemented. This means that the ISO requirements regarding determination of Quality Policy, responsibility and authority, etc. are fulfilled and in addition that procedures exist for document control, nonconformity treatment, corrective actions, quality records, internal quality audits, training, etc. In this chapter, we will therefore concentrate on those elements that are particularly relevant in engineering and design of concrete structures. The general requirements for Quality Assurance in engineering and design are described in the following ISO standards: • ISO 9001:1994, item 4.4 • ISO 9004–1:1994, item 82. In general, the ISO 8402:1994 terms have been used. In the cases where these deviate from the terms that are used among concrete professionals, the latter have been used. 6.3.2 Some important QA-elements and QA-tools in engineering and design
“QA-elements” in this context means the procedures, etc. that are required for the Quality System to satisfy the 20 main requirements in the ISO 9001:1994 standard. These elements have been given the numbers 4.1 to 4.20 in the standard. “QA-tools” are used as a less precise term for certain specific techniques and tools that will contribute to fulfilling the requirements effectively. Some of those elements and tools have been described in brief below, namely qualifying key personnel, Self-Check, Discipline Check (DC), Inter Discipline Check (IDC), Third Party Verification, Design Review (DR), Hazard and Operability Studies (HAZOP), Worst-Case Analysis and Quality Audit. Fuller descriptions are given in Appendices B-H.
2
Note that ISO 9004–1 is a Guide; it is not intended as a contract requirement.
(a) Qualifying key personnel In design of concrete structures, as in other high technology projects, one of the most important preventive measures is ensuring that fully qualified personnel are assigned to the key positions. Before anyone is appointed, it should be made clear which basic qualifications and what type of experience, for example from similar projects, are required. Subsequently, the most suitable candidates should be selected based on CVs, references and interviews. Finally, the candidates should be asked to outline how they will attack the task, and the answers evaluated. (b) Self-Check All analyses, calculations and documents should be thoroughly self-checked, that is to say checked by the person or persons who have carried out the work, before the results are passed on to others. As mentioned, the policy should be to make everything right the first time. Fig. 6.1 shows the relation between Self-Checks and the other elements in the verification process.3 When Fig. 6.1 is read vertically, the order of the different checks appears. Thus, all documents and drawings should first be subject to a Self-Check, i.e. a check by the same person who is carrying out the task. If a nonconformity (NC) to the specification for that particular task is observed it should be corrected at once, i.e. before the document or drawing is passed on. The relevance of the subsequent checks depends on the criticality of the document or drawing. If the task has no criticality classification at all, the result would pass on to a Design Review (if required) or directly for approval. If classified I, II or III (see Table 6.2) the document or drawing would be subject to a Discipline Check at a suitable level. Documents or drawings of Criticality II and III would in addition be subject to an Inter Discipline Check (if relevant). The most critical results (Criticality III) would finally be subject to a Third Party Verification as well. If nonconformity is detected during these checks, the document or drawing will, of course, have to be corrected or re-worked. In Fig. 6.1 this is indicated by the dotted feed-back loops. Design Review, Discipline Check, Inter Discipline Check and Third Party Verification are more closely examined below and in Appendices B—E. (c) Criticality The amount or level of quality assurance of documents and drawings should reflect their importance (level of criticality), which could be decided by a qualified person or by a Design Review. The criticality should be clearly stated in the document. This is to ensure that the level of attention coincides with the document’s importance in relation to quality, safety, environment and/or economy. Three levels of criticality are described in Table 6.2. In engineering and design of Gravity Base Structures (GBS’s) a major part of the technical calculations, documents and drawings will in principle be of Criticality III. However, as indicated in Fig. 6.1, there are still several possibilities for tailoring the amount of quality control to a suitable level. (d) Discipline check (DC) A Discipline Check is an inspection to ensure that the technical documentation satisfies all internal and external requirements within one’s own discipline before further distribution and use. In order to be acceptable, a Discipline Check has to be independent, i.e. carried out by a competent person or body other than the one(s) that drew up the documents, or were responsible for or involved in the process in some other way. This goes for the other types of
3
The term verification is used here as a joint term for several types of checks that aim at providing evidence that specified requirements has been fulfilled.
verification as well, except for Self-Check. However, as mentioned above, the necessary level of independence may vary according to the criticality of the document or drawing.
Table 6.2. Suggested criticality classification of documents and drawings4
(e) Inter discipline check (IDC) An Inter Discipline Check is an inspection and a review of technical documentation to ensure that it fulfils all internal and external requirements and considerations for other (i.e. interfacing) technical disciplines before further distribution and use. The check should be carried out by competent persons from the relevant other disciplines. An Inter Discipline Check is an independent check. It comes after, and in addition to, SelfCheck and Discipline Check (see Fig. 6.1). It is carried out only when an interface to other disciplines exists. The Inter Discipline Check has been further described in Appendix C. (f) Third party verification/External verification A Third Party or External Verification of documents and drawings consists of Document Review (Level 1), Extended Document Review (Level 2), Independent Calculations (Level 3) or Scale Test (Level 4) carried out by a different company or organization than the one responsible for the executed work (Fig. 6.1). Third Party Verification may also include external Quality Audits to verify that the quality system is effective. From the authorities’ point of view, the main aim of Third Party Verification is to obtain objective evidence that the requirements have been met. The operator could therefore be instructed to carry it out. From the point of view of the operator and the personnel involved in engineering and design, it is important that the Third Party Verification is synchronised with the design work. The reason behind this is that the verification can and should give effective support and current corrections to design. This has been further explained in Chapter 7, Verification of design.
4 Note that several publications and companies use the opposite order, i.e. Level I is the most critical, Level III the least critical. The reason for the suggested order in this book is to match the Levels 1–4 in Fig. 6.1
6.3.3 Quality Assurance elements related to the main activities in engineering and design Fig. 6.2 gives a general view of the main activities and documents in the engineering and design phase.
evaluated at the right organizational level before they are implemented. This implies among other things that the person(s), who worked out the original requirement or the previous solution, should have an opportunity to assess the consequences of the proposed changes, before they are accepted and implemented. When a change has been accepted, everyone who is in position of the original documents must be informed of the change at once. During the review and detailing of the Design Basis for the different disciplines, SelfChecks, Discipline Checks, and Inter Discipline Checks should be carried out. This is indicated with small parallelograms in Fig. 6.2. A (Project) Quality Plan should be prepared at the same time as the review and detailing of the Design Basis (see Fig. 6.2). The Quality Plan describes the particular quality assurance activities that are to be carried out in the project to satisfy the requirements, and what resources should be used for this. Table 6.3 gives an example of the layout of a Quality Plan for the engineering and design phase. The Quality Plan should be rooted in the operator’s Quality System and be co-ordinated with or constitute part of the Project Plan. The Quality Plan for the complete project should include the following elements as well (ref.: ISO 9004–1:1994, item 5.3.3 and ISO 9004–5 Guidelines for quality plans): • Description of aims for quality • Distribution of responsibility and authority in different project phases • Procedures, methods and work instructions that are to be employed Table 6.3. General layout of a Quality Plan during engineering and design (example)
• Programs for testing, reviews, inspections and audits in the different phases • System for change and modification of the Quality Plan as the project proceeds • Other initiatives that are to be carried out in order to reach the goals. The Design Brief (see Fig. 6.2) is prepared on the basis of the revised Design Basis. The Design Brief lays down the Design Criteria further, together with the design procedures, important interfaces and preferred methods for analysis and calculations. Other important parts are the results of criticality evaluations for the individual parts of the structure, and a specification of the software programs accepted for use. During development of the Design Brief, the same quality elements as in the treatment of the Design Basis should be employed, that means Self-Check, Discipline Check, Inter Discipline Check and Design Review. Since the extent of the documents is steadily increasing as the project progresses, it is evident that the checks require more and more resources. To some extent, different kinds of competence are needed, too. The analyses are carried out on the basis of the Design Brief. They consist of Dynamic Wave Analysis, Stability Analysis and Global Analysis. Dynamic Wave Analysis results in dimensioning waves for different parts of the structure, hydrodynamic loads, masses and damping. The results are used as basis for the Global Analysis. Stability Analysis gives an answer to whether the structure has the necessary stability against capsizing during the construction process, submerging, tow-out, installation, operation and service. Both floating stability and geotechnical stability has to be assessed. Global Analysis gives an answer to what kind of strain the different parts of the structure is exposed to during submerging, tow-out, installation and operation. The three above-mentioned analyses together make up the basis for the later dimensioning, and are therefore critical for the structure’s safety and fitness for use. Related to Figs. 6.1 and 6.2, there is a need for a more comprehensive quality assurance than previously, in this phase of the project. In addition to Self-Check, Discipline Check and possibly Inter Discipline Check, Third Party Verifications would therefore be needed and hence required by the authorities. The dimensioning is carried out on the basis of a general view of the load effects. During dimensioning, the length and location of the reinforcement, the final wall thickness and the quality of the concrete and the reinforcement are determined. The result of the dimensioning is entered in the Dimensioning Document. This should be subject to Self-Check, Discipline Check and possibly Design Review before the drawing work starts. The drawing work will result in drawings, material lists and descriptions that are necessary for the construction to start (see Fig. 6.2). All sketches and specifications should be subject to Self-Check and suitable independent verification. A Design Review (“Engineering Readiness Review”) should be completed before release for construction and fabrication.
7 Verification of design
Tore H.Søreide, Reinertsen Engineering
7.1 Introduction The purpose of this description of verification is to present a procedure for control for detailed design of offshore structures. For exemplification, Chapter 7 refers to government regulations from the Norwegian Petroleum Directorate (NPD, 1997) as well as company specifications from the Norwegian oil company Statoil (Statoil, 1991). Both documents require design verification to be part of the quality assurance. The major activities of verification are outlined below, together with the basis for verification in the form of authority and company specifications. Chapter 6 has presented the overall system for quality assurance, where design verification is an integrated part. The objective of verification is to guarantee that the final product is in accordance with the Government regulations and Company specifications. For engineering, the outcome is in the form of drawings and specifications for fabrication. The NPD regulations (NPD, 1997) give detailed requirements concerning verification. Section 7.2 discusses these requirements as well as the corresponding verification activities. Depending on their importance for the quality and safety of the final structure, the various documents need different types of control. Section 7.3 presents four levels of verification that involve document control as well as prototype testing. Section 7.4 considers a system for external verification. Special emphasis is given to the view that the verification should ideally also be a support for engineering, so that time coordination becomes important. Alternative means of organizing verification are presented in Section 7.5, while Section 7.6 considers administrative services coupled to the work on verification. These services include budgeting and a set-up for reporting and communication between the engineering and verification bodies on unsolved topics. Qualification requirements for the engineers participating in the verification work are dealt with in Section 7.7. Special effort should be made to guarantee that there are particularly competent people in the technical lead position within the verification activity. An essential function of the technical leader is to sort out the major technical questions for follow-up. Section 7.8 exemplifies the scope of work involved and how the verification job may be planned. Fig. 7.1 illustrates the importance of co-ordination in time between engineering and verification. It is clear that for the verification to affect design prior to fabrication, the comments from verification should reach engineering within the timeframe when the problem is being addressed. If not, it is often difficult for engineering to react to the comments from verification, and a negative atmosphere of dialogue arises.
7.2 Norwegian Petroleum Directorate requirements The NPD regulations on loadbearing structures (NPD, 1997) formally states the independence between the engineering and verification activities within the same project, emphasizing a third-party verification. The operator is to consider the amount of verification that is related to the critical aspects of the structural part under engineering.
NPD provides a detailed list of verification activities. These are areas of verification that all engineering on offshore loadbearing structures should undergo. Among the essential areas listed are: • • • • • • • • • • Design Basis in accordance with guidelines Qualifications by personnel in engineering Organization of engineering Documentation and testing of computer programs Load modelling Response analysis Capacity control Tolerances in design Drawings in accordance with design calculations Design and forming of details.
The NPD requirement (NPD, 1997) regarding tolerances and inaccuracies as part of verification, relates to the check that the choice of material safety factor in design is in accordance with the Design Basis specifications, as well as with the system of control during fabrication. For critical areas with complex geometry, the verification of capacity should include a sensitivity study, in which upper limits for deviations are considered, even beyond the tolerances specified. Non-linear analysis is often an alternative to linear elastic analyses for complex stress flows, representing a more realistic simulation of stress redistribution. The verification of fabrication drawings is to pay full attention to the control so that the drawn structure is in accordance with the input and the results of the engineering calculations. Major elements for control are the dimensions given, as well as reinforcement amounts and locations. The amount of pretensioning specified for fabrication should also be checked against the corresponding load modelling. The term “D-regions” (D for Discontinuity) is used to locate areas for design that the global analysis model does not cover, and where special calculations of capacity are needed. The verification of these areas either is made by a local finite element model, or, in some cases, where ultimate capacity is to be verified, by a strut and tie model. The verification of D-regions puts special requirements on the personnel. Experience in practical design may be needed to be able to determine the flow of forces in and out of the detail in question. Section 7.8 gives some proposals on verification activities that follow from the NPD regulations. However, the complete set of verification activities depends on the problem faced, and, thus, a verification plan should be initiated at the start of all engineering projects. 7.3 Levels of verification 7.3.1 Choice of levels
As discussed above, the level of verification depends on how critical the actual engineering activity is. As a general rule, the major analysis elements are given top priority. It is convenient to classify the verification into the following four levels: Level 1: Level 2: Level 3: Level 4: Document Review Extended Document Review Independent Analysis Scaled Model Tests
A description of objectives and content is given below for the above four classes of verification. 7.3.2 Level 1: Document review
strength parameters. The evaluation of the analysis model is to be based upon separate considerations concerning load-carrying behaviour, including possible dynamic effects. The process of document review is at least to be applied for all basic documents in analysis and design procedures (Design Basis and Design Brief). For these documents, the document review level 1 of control is made even for documents that are very critical. 7.3.3 Level 2: Extended document review
The extended document review form of verification implies level 1 control of a document, supplemented by a check of calculations. The control is still related to a specific document from engineering, however, essential calculations are checked either by simplified hand techniques or by local finite element models. All control calculations are to be stored together with the document. They should be clear and easy to follow in the case of later technical discussions. Normally, the level 2 control calculations are not reported separately. Level 2 control is to be made for all major documents on analysis and design, that are not covered by independent analyses in design. 7.3.4 Level 3: Independent analysis The verification by independent analysis is a completely separate analysis of the total structural system, or alternatively parts of it. It is a general rule in practice that independent analyses are made by a different program system than the one applied in engineering. The different runs on independent analyses are to be co-ordinated, so that in total they cover the main activities of analysis and design. The independent verification analyses are to be reported by separate verification documents, and comparisons with engineering results are to be included. 7.3.5 Level 4: Scaled model test
The experimental verification by scaled model tests normally deals with critical details in the structure, in most cases also in reduced scale. The objective is to verify analysis models on capacity, or to check the feasibility of fabrication. Model tests are normally supplements to the verification of calculations, as described in previous chapters. Strict requirements are to be put on planning, execution and evaluation of such model tests, especially on the effects of scaling. Test laboratories thus normally should carry out this type of verification. 7.3.6 Choice of verification level
An activity plan for the verification is to be made, as well as scope of work for each activity. The extent of control, together with computer programs for control calculations are outlined here, also see Sections 7.6 and 7.8. In general, the following choice of verification levels should be made: Level 1: Design Basis document Design Brief document
Loads Criteria for capacity Level 2: Secondary calculations Internal verification Finite element models Local capacity controls Interface analysis/capacity control Load models Design waves Global stress analysis Load combination Capacity control Global response (wind and waves) Capacity of vital details Fabrication feasibility of details in full scale (reinforcement).
Level 3:
Level 4:
7.4 External verification 7.4.1 Alternatives in external verification
The operator gets a key role in the clarification effort on comments from verification. The heavy involvement of the operator in the technical discussions between engineering and verification also represents a large risk for time delays, and the gains from verification to engineering may disappear. Alternative 2: Direct communication between engineering and third party verification An alternative system for verification of engineering is illustrated in Fig. 7.3, involving direct communication between engineering and verification. The formal responsibility is still by the operator, as well as the task of discipline co-ordination. The main difference from the scheme in Fig. 7.2 is that direct technical discussions now take place between engineering and verification. 7.4.2 Choice of alternative
The criteria for the selection of system for third-party verification are as follows: Criterion 1: Technical qualifications The sum of experience and theoretical basis by the operator and the third-party verificator for alternative 1, is to be compared to the qualifications by the verificator for alternative 2. The main emphasis is laid upon key persons for the verification, their formal education as well as experience from relevant projects. Criterion 2: Plan for third-party verification A scope of work for verification is to be set out, including an activity plan and time schedule for reporting back to engineering. In the selection of company for third-party verification, the above activity description should be a major criterion. Criterion 3: Cost Based on an estimate for man-hours a budget is to be made for the verification work. The operator may choose a split contract based on separate activities, or a type of framework agreement, with an upper limit on costs. As seen from the above alternatives, the type and size of project are quite often decisive for the choice of system for verification. In the case of operator supplement to third party verification, the alternative in Fig. 7.3 should be chosen. For both alternatives 1 and 2, the verificator is to report to the operator the work done. In the case of alternative 2, lists of non-conformances are to be sent in parallel to engineering and to the operator, and the operator may supplement this by comments. 7.5 Internal verification The internal verification of results from engineering is to be included in the plan for verification, including personnel, level of control as well as technical subject for control.
Figs. 7.2 and 7.3 each shows a box for internal verification in engineering. The plan for verification should co-ordinate this activity with the overall set-up for the third-party verification. The most common system is to have a separate group in engineering, that functions like a group for third-party verification. All comments and the follow-up of comments from internal verification are then to be documented. The alternative is that well-qualified personnel come into the engineering team with their major effort on control. Communication to engineering is easier, and internal verification in engineering can now also become a daily support for the design work. It is, however, still essential to keep the formal requirements intact, such as reporting of comments and documentation of the follow-up, even though the personnel doing the internal verification are in close contact with engineering. Both the above alternatives involve the internal verification being an independent activity from engineering, so that the personnel for verification themselves do not have responsibility for design work. 7.6 Budgeting, reporting and follow-up of non-conformances 7.6.1 Budgeting This section presents a system for the budgeting of the third-party verification activities. It also describes forms of reporting by verification as well as follow-up of non-conformances. Prior to the start of verification, a detailed plan is to be made, taking into account the control activities in verification as well as the treatment of non-conformances. For the case of a split contract for third-party verification, or alternatively with a framework agreement, a budget for the verification work is to be made. The budget schemes contain the scope of work for each verification activity, together with planned man-hours and personnel. A similar set-up should also be made for planning additional work during verification, either as an extension of existing activities, or by making a new scope of work. Any exceedance of the budget has to be reported immediately and be subject to verification. Possible modifications of scope of work, so as to reduce the costs, need acceptance by the operator. 7.6.2 Reporting of non-conformances
The comments from verification are to follow the document. In Fig. 7.5 the list from verification contains the reference to engineering document, numbering of non-conformances as well as a technical description of each comment, possibly also supplemented by sketches. A column is also included for reporting on the follow-up, in which a signature confirms and dates the verification of the corrective action, when agreement between engineering and verification is obtained. Each list on non-conformances is to be checked and signed by the technical supervisor for verification.
7.7 Requirements concerning qualifications 7.7.1 Documentation of qualifications Necessary qualifications are to be documented for the persons making the verification, in the form of formal education as well as relevant experience. 7.7.2 Requirements for technical leader for verification
A technical responsible person is to be identified by the third-party verificator. The primary function of the technical leader is to evaluate all comments from verification to engineering, assuring that relevant comments are forwarded. A wide theoretical basis is necessary to cover global behavioural effects on the structure, as well as local problem areas. As a minimum, ten years of experience in structural design should also be required in order to lead the verification of larger projects. The function of the technical leader is to evaluate the technical activities presented for verification and distribute these among the engineers for control. To follow up the control work, knowledge is also needed concerning special analyses on load effects, response analysis and design. The technical leader for verification is to check all comments that arise during the control work and sort out the essential non-conformances that are to be forwarded. The comments also ought to be ranked with respect to critical aspects, making the discussion with engineering rational. At certain milestones, the technical leader for verification is to report on the status of progress, highlighting on the critical items that are still not cleared from the list of nonconformances. Evaluations are then to be made whether the items still remaining require a redesign and thereby a stop in fabrication. The technical leader represents the verification in all meetings with engineering and the operator. Qualifications in technical presentations, also in foreign languages, are a requirement in larger projects. 7.7.3 Requirements for engineers
For the verification engineer in general, good basis within structural mechanics is necessary, including theory concerning load effects, stress analysis as well as capacity control. It is essential that tasks for the verification engineers are related to their qualifications. It is the responsibility of the technical leader for verification to let the engineers have the relevant documents for control. In the case of level 3 control by independent analysis, experience in the use of computer programs is required. Further, structure behaviour knowledge is needed for making self control of the level 3 analysis results, prior to reporting on possible non-conformances.
7.8 Scope of verification activity 7.8.1 Planning of verification The present section outlines the plans for some major activities within verification, the objective being to come up with major issues in the verification of documents from engineering. It is, however, to be emphasized that the activity schemes below do not represent the complete set of verification activities for a structure design project. 7.8.2 Verification of global analysis model
The control of the global model for stress analysis has its main objective to verify that the finite element mesh represents the load carrying behaviour of the structure. The verification is also to identify areas where the model is not representative for the real structure behaviour and where extra effort on design needs to be taken. The control of the model follows the level 2 scheme of verification. The document on global model by engineering is to have plots on section stress resultants and verification ought to check these with simplified techniques. Tests are to be made on the accuracy of the model in areas with high degrees of element distortions, ending up in a documentation of the feasibility of the analysis model to handle certain complicated areas. As a supplement to the document control of the global analysis model, an independent analysis of stress results is to be performed by verification, applying an independent computer program. The modelling should include an evaluation of areas where separate analyses are needed (D-regions). The independent analysis model is related to the model from engineering, together with comparisons on stresses for design. A separate document is made on the verification of analysis model. 7.8.3 Verification of design waves
extremes of a stochastic analysis. The process of design wave generation is made for a certain number of structure areas. The control of design waves from engineering should also include a check of the correct design wave being applied for the structure part for which it is valid. Based on the independent analysis control of stress resultants, a separate verification is made of capacity and reinforcement amounts. References Norwegian Petroleum Directorate (NPD), (1997) Acts, Regulations and Provisions for the Petroleum Activity. Norwegian Petroleum Directorate, Stavanger, Norway. Statoil (1991) Structural Design for Offshore Installations, Specification for Design. N-SD001, Statoil, Stavanger, Norway.
Fixed platforms—Concrete substructures checklists The objective of Appendix A is to present possible checklists for design of offshore concrete substructures. Such checklists could be useful tools for the practical design to avoid important steps being missed out during the different stages of the design process.
The following key relates to checklists A1 through A5:
Discipline Check (DC)
Discipline Check is a check to ensure that technical documents satisfy all internal and external requirements within one’s own discipline before further distribution and use. A competent person, other than the one(s) that drew up the documents should carry out the check. To prevent defects and additional work in later phases due to the documents not being based on the correct assumptions or limitations, or that they contain technical or formal defects or weaknesses. 1. Discipline Check does not guarantee that the interfaces to other disciplines are attended to. For this purpose, Inter Discipline Check (IDC) is used. Discipline Check can be carried out by a person (or several persons) who has the same, or a higher level of competence than the person(s) who worked out the documents. DC adds safety in comparison to Self-Check, but does not in principle, due to the above-mentioned reason, attend to other defects or weaknesses than those that could have been discovered by a more detailed Self Check. ISO 9001, item 4.1.2.2 ISO 9001, item 4.4 ISO 9001, item 4.5.2 Norwegian Petroleum Directorate (NPD): Regulations for structural design of loadbearing structures The individual Discipline Manager is responsible for carrying DC into effect within his own area of responsibility before the documents are distributed to others. If consultants are used the relevant Discipline Manager, if necessary supported by the QA department, is also responsible for ensuring that DC has been carried out at the consultant’s. DC is carried out by checking that all internal and external requirements have been attended to, particularly the following: a) Accordance with the last, valid version of the Design Basis (Design Baseline), which lays down the loads, environmental data and other functional requirements, conditions for use, geotechnical conditions, reference documents, etc. that will make up the basis for the engineering and design work. b) Accordance with the last, valid version of the Design Brief, which describes how the engineering and design work should be carried out (that is to say what important analyses,
calculations and checks are to be carried out), the Design Criteria, the design procedures and the interfaces. Check to ascertain that all analyses and calculations are based on the correct conditions and limitations, that the applied software has been approved of for its current use, that all analyses and calculations are carried out correctly and that the results seem reasonable. Check of all analyses and calculations by means of alternative methods and software, if required. This goes especially for, but is not limited to: * critical parts of the structure (e.g. tri-cells) * cases where new (i.e. unverified) methods or solutions are used Check to ascertain that relevant experience from other projects has been taken into consideration. Check to ensure that all interfaces within the discipline has been attended to. Check to ensure that all formal requirements to the documents have been met, including identification, readability, clarity, neatness, references, dates and signatures. Check to ascertain that all drawings are in accordance with the calculations, that geometrical measures are in mutual accordance, that critical measures and issues have been especially marked (“flagged”) and that the structure is construction- and service friendly. Check to ensure that interfaces to other disciplines has been attended to, as far as possible. (See in addition, Inter Discipline Check).
Records: References:
All inspected documents and completed checklists should be dated and signed by the person(s) who carried out the check. * * ISO 9001, item 4.4 ISO 9004–1, item 8
Inter Discipline Check (IDC)
Check to ensure that technical documentation satisfies all internal and external requirements with regards to other technical disciplines, before further distribution and use. Preventing defects, unfortunate solutions or extra work from occurring, due to lack of consideration of interfaces to other disciplines. Inter Discipline Check in principle presupposes that Discipline Check (DC) has already been carried out. If this is not the case, special initiatives are required, to meet the purpose of IDC. For IDC to be effective, it is important that the relevant disciplines give the work the necessary priority. The individual Discipline Manager is responsible for this. * * * * 1. ISO 9001, item 4.1.2.2 ISO 9001, item 4.4 ISO 9001, item 4.5.2 Norwegian Petroleum Directorate (NPD): structural design of loadbearing structures Regulations for The individual Discipline Manager is responsible for sending the documents to other disciplines for IDC, in accordance with the project’s Quality Plan and the prevailing procedures and distribution list. Normally, Discipline Check should have been carried out first. If IDC is to be carried out in parallel to DC, this should be made evident, and necessary initiatives carried out to prevent mistakes and problems from occurring. A general Checklist, information about possible particular conditions that should be assessed and a fixed deadline for comments should accompany the documents. The internal procedures regarding document control must be followed regarding dispatch, recording and filing throughout the IDC-process. The Discipline Manager’s responsibility also applies when consultants work out the documents. The personnel carrying out the IDC should follow the Checklist and make comments. The Checklist should contain, but are not restricted to, possible conflicts, problems and indistinctness regarding: • • interface to other disciplines construction techniques and project progress
contract and internal decisions possible particular requirements formal requirements to the documents (identification, readability, clarity, references, dates and signatures)
The Checklist, Distribution list and possible document pages with comments are dated, signed and further distributed in accordance with agreed procedures. The Discipline Managers receive the comments and evaluate their relevance before passing them on to the person who worked out the documents. The Discipline Manager must approve of the implementation, possibly the neglecting of the ideas and comments. Disagreements regarding technical issues should be solved at meetings with the involved parts. If agreement cannot be reached, the case should be sent one level up in the organization to be decided on. If the comments lead to great changes, a new IDC must be carried out.
5.
6. Records: References:
All documents and filled in checklists should be signed and dated by the one(s) who carried out the check. * * ISO 9001, item 4.4 ISO 9004–1, item 8
Verification
Verification Confirmation by examination and provision of objective evidence that specified requirements have been fulfilled. [ISO 8402:1994] Notes: 1. In engineering, design and dimensioning, verification concerns the process of examining the result of a given activity, to establish conformance with the stated requirements for that activity. Verification is thus a joint term for several elements of Quality Assurance which concern different types of internal and external reviews, checks, inspections, tests, alternative calculations, surveillance’s and quality audits. 2. The term “verified” is used to designate the corresponding status.
Objective evidence Information, which can be proved to be true, based on facts obtained through observation, measurement, test or other means. Independence The verification is independent when personnel other than those who are directly responsible for the work and the results that are to be verified carry it out. Furthermore, the personnel should not report to the same manager and they should be free from any pressure that may influence on their judgement. Internal verification Verification carried out by own employees. External Verification/Third Party’s Verification Verification carried out by personnel employed by and reporting to another organization/body. Purpose: The purposes of all types of verification are the following: 1. Preventing defects and failures in the final product or service, as well as preventing additional work and costs due to nonconformities being discovered at a later point in time. Providing evidence of, and thus greater confidence in, that the requirements have been met, and that the product will be well fit for use. The final proof of the product’s fitness for use can only be obtained by real use. The applied technology and the verifying personnel’s competence in each case limit the value of the verification activities.
The degree of independence is often of vital importance to the confidence in the results of the verification. Internal verification is necessary, but not always sufficient, for instance with respect to the authorities and one’s own management. Usually, the reason behind this is not suspecting occurrences of conscious actions or omissions, but rather the fact that the ability of someone discovering defects in his/her own work is inferior to someone “from outside”. Besides, one cannot ignore the fact that the power of judgement could be impaired due to stress, for instance regarding time, money or the mere knowledge about potential consequences of discovering defects and nonconformities. An example of the latter would be if costly analyses or calculations had to be redone if errors were discovered.
Example 1. The value of a simulation depends on how well the computer program has been tested (verified) for similar tasks previously, and that the operator handles the data and the program correctly and unaffected by the desired result. Example 2. A Design Review could be an efficient means of ensuring that the previous experience and the total competence of the organization are conveyed to the structure. However, a condition for this is that persons who have the adequate competence and experience carry out the review, and that sufficient time has been allocated for the purpose. Example 3. When calculations which have been carried out by means of Finite Element Analysis (FEA) are to be verified, a different method should be applied, or at least a different program. The reason behind this is to avoid the same (systematic) mistakes from being repeated in the verification. In addition, it is presupposed that possible nonconformities, which have been discovered by the verification, will be subject to an indepth analysis and assessment. If such nonconformity were explained away, for instance by claiming that a coarser model was used in the verification compared to in the original calculations, the verification would give a false feeling of safety. Reference requirements: * * * * * 1. ISO 9001, item 4.1.2.1 ISO 9001, item 4.1.2.2 ISO 9001, item 4.4.7 ISO 9001, item 4.4.8 Norwegian Petroleum Directorate (NPD): Regulations for structural design of loadbearing structures Both internal and external verification should in principle be independent, that means carried out by personnel other than the person(s) who is (are) directly responsible for the work that is to be verified.
Regular verification should follow predetermined procedures. Independent, external experts are often used in the verification of high-technology activities or products. These experts should in principle be free to choose their own methods. Neither the project team nor others should direct the verification process in detail, for two reasons. First, because this often creates confusion regarding authority (who is responsible for what) and second, because it can reduce the confidence in the results of the verification. The verifying party in addition ought to have a major impact on which activities or results should be subject to (third party) verification. Verification during engineering, design and dimensioning can be carried out on four different levels (see Fig. 6.1): Level 1: Document Review Level 2: Extended Document Review Level 3: Independent Calculations Level 4: Scale Test(s).
4.
Document Review involves a check to ascertain that all documents from the engineering and design phase (calculations, specifications, drawings, technical reports, etc.) are available, and impeccable, unambiguous and fit for use. The review could be carried out on all or some of the documents, depending on the criticality. Document Review is not likely to reveal more basic defects, e.g. due to the use of inferior methods. Extended Document Review implies normal Document Review supplied with checks of selected items. Those checks should be documented and filed together with the original document. Independent Calculations should be carried out if the consequences of potential defects or nonconformities are major. The highest level of safety, and thus the greatest confidence, is achieved if a different method, computer program, computer, etc. is used. However, this is more time consuming. It is also required that methods, computer programs, etc. that are used during verification are themselves verified for the current application. Simple analyses and manual calculations can in some cases be an efficient, cheap and sufficient verification of the results. Independent calculations should concentrate on the most critical parts of the structure. Scale Test(s) imply that selected parts of the structure are built, usually on a smaller scale, and loaded or otherwise tested, under controlled conditions. The purpose could be to verify in practice that the structure is able to take the loads it has been dimensioned for, with the given margins for safety, and/or that it can be constructed and inspected with the
presupposed methods, tools, dimensions and materials. Due to practical and economic reasons, such tests with large concrete structures usually cannot be carried out on a full scale (1:1), When evaluating the results, it is of vital importance to take possible scale effects into consideration. References: * * * ISO 9004–1, item 8.5 ISO 9001, item 4.4.7 ISO 9001, item 4.4.8
Design Review (DR)
Documented, comprehensive and systematic examination of a design to evaluate its capability to fulfil the requirements for quality, identify problems, if any, and propose the development of solutions. [ISO 8402:1994] Exploiting the total experience from previous projects to achieve an optimal structure. It is, of course, particularly important to prevent serious faults, which can lead to structural breakdown. 1. 2. Design Review is not alone sufficient to ensure a high quality structure. The benefits of DR depends on the participants in the review, particularly their competence, experience from similar projects and structures, creativity and ability to identify potential problems. The Design Review is usually of advisory character. The effect therefore also depends on the project leader having the will and ability to take the advice into consideration.
Purpose:
Limitations:
3.
Reference requirements: • • • • Principles: 1. NPD: Regulations for structural design of loadbearing structures NPD: Regulations concerning the licensee’s internal control in petroleum activities ISO 9001, item 4.4.6 NPD: Regulations concerning implementation and use of risk analyses in petroleum activities
Design Reviews can in principle be carried out in all phases during engineering, design, dimensioning, construction and fabrication. The reviews should be included in the Project Plan or Quality Plan. Ad-hoc-like reviews should be arranged if considerable changes in the functional specifications arise, or if needed due to particular problems. To characterise the individual reviews further, supplementary designations are often used, for instance: • • • • Preparatory Design Review Following-up Design Review Design Review of the Design Basis (Design Baseline) Design Review of the Design Brief
Design Review of the Design Criteria Civil Design Review Design Review of the Shaft Design Design Review of tri-cell reinforcement, etc. Mechanical Design Review Engineering Readiness Review (often used before permission to start the construction work is granted), etc.
The project leader, or the person he/she has authorised, is responsible for preparing, summoning and leading the meetings, in co-operation with the QA department. The participants, sometimes called the Design Council, should be persons with relevant competence regarding the themes that are to be dealt with. Consultants, the authorities and/or contractors may be invited, when appropriate. The necessary documents to be reviewed should always have been studied in advance. Those themes which should be dealt with could include the structure’s fitness for use, constructability, testability, strength, reliability, maintainability, safety, environmental considerations, life cycle costs, etc. The result of DR could be: • calling attention to nonconformities, weaknesses and potential problems with the proposed solution, for instance in light of experience from other projects proposals for improvement confirmation of favourable choices for solutions agreement on the need for other reviews.
3.
• • • 4.
Minutes of the meetings should be drawn up. It is important to give the reasons for the advice and recommendations, as well. If the project manager chooses not to follow the advice, the reason for this should also be recorded. • • • • ISO 9001, item 4.4.6 ISO 9004–1, item 8.4 ISO 9004–1, item 8.6 ISO 9004–1, item 8.7
Hazard and Operability Analysis (HAZOP)
Formal, systematic and critical review of different parts of a system, design or structure in order to identify potential safety and operational problems. Identifying possible safety and/or operational problems that may arise during construction, operation and maintenance of a process plant or a structure. The analysis could be a complete risk analysis or a prestudy for later, more detailed studies of certain critical parts of a plant or structure. 1. 2. 3. 4. Any non-predicted hazard will not be part of the analysis. The results depend on the competence of the analysis group. HAZOP does not take human failures and common cause failures into account. It is difficult to identify component failures and environmental effects as reasons for nonconformity. NPD: Regulations for structural design of loadbearing structures NPD: Regulations concerning the licensee’s internal control in petroleum activities NPD: Regulations concerning implementation and use of risk analyses in the petroleum activities
Limitations:
Reference requirements: • • • Principles:
HAZOP may be carried out as part of pre-engineering and/or detail design, as well as in connection with maintenance and/or modifications of the structure, operation procedures, etc. The analysis can be divided into 5 steps: 1. Define the purpose of the study, the methodology and the time schedule. 2. 3. 4. 5. Select the members of the analysis group. Prepare the analysis work. Carry out the analysis. Record the results.
The analysis group should be made up of persons with different backgrounds, who have special competence within their own field. The group usually consists of a leader, a secretary plus 4–6 additional persons, depending on the size and the complexity of the object of the analysis. If there is a need for it, other persons can be brought in. The work is mainly
carried out in a series of brainstorming meetings, conducted by the leader. The process demands much from the leader of the working group; he or she ought to have previous experience with HAZOP. The analysis is normally done on the basis of drawings, construction procedures, etc. The analysis work starts by clarifying the purpose and the normal condition of the elements that are to be evaluated. The work is directed by the HAZOP-leader by means of a series of guiding words and checklists. Examples of guiding words are “none”, “bigger than”, “smaller than”, etc. Every guiding word is used on different structure elements at the specific items that are to be examined. By means of these guiding words, possible nonconformances from the normal condition in every structural element are identified, as well as the reason(s) behind nonconformity and the consequences. The analysis should be carried out for different conditions. The results should be verifiable and recorded by means of a HAZOP Report Form. The form should as a minimum contain a column for guiding words, nonconformances, causes, consequences, recommenda tions and comments. The latter could, for example, be questions for the project manager, recommendations regarding changes in design or comments about particular risks that ought to be dealt with in special procedures. References: * * NS 5814:1991 Requirements to risk analyses Rausand, Marvin, 1991: Risikoanalyse—Veiledning til NS 5814, Tapir, Trondheim. (This is a guide to the Norwegian Standard NS 5814.)
Worst-Case Analysis
Systematic analysis and assessment of the consequences of the worst possible input data, occurrences and combinations of occurrences for people, the environment and assets. 1. To verify that safety and other important functions are maintained under abnormal loads, foreseeable abuse and abnormal human stress. To ensure that possible decisions on not dimensioning for such extreme conditions are made on the right (i.e. a high enough) level in the organization.
Purposes:
2.
Limitations:
In worst-case analyses, only those occurrences and combinations of occurrences which have been identified beforehand, and assessed to be of current interest to such analyses, are dealt with. * 1. NPD: Regulations for structural design of loadbearing structures Worst-case analyses should be carried out for critical design parameters to obtain a general view of the consequences for personnel, the environment and material values due to extreme stresses that may occur in connection with testing, operation and maintenance. The environment may cause such extreme stresses, by single occurrences or by combinations of occurrences. The assessment may start out with the normal condition of the installation. Alternatively, potential nonconformity’s can form the starting point. For example, the normal condition would be that all reinforcement has been installed as planned, while a potential nonconformity might be that 5% of the reinforcement is lacking or seriously corroded in critical sections of the structure. In order for worst-case analyses to be carried out, there ought to be a certain probability for the occurrence(s) to happen. The identification of those occurrences and conditions should be based on a systematic review of the installation. The worst possible occurrences might be analysed one by one, or as a combination of occurrences. Such combinations should be possible, although not very likely to happen at the same time. The analysis of combinations could be by means of a systematic review of different scenarios, e.g. in the form of a matrix. Worst-case analyses do not use a particular technique or method; it is rather a philosophy for finding out how robust the installation
is under the influence of extreme conditions and stresses. The principle can be used both in risk analyses and also for instance in stress calculations and dimensioning. 5. The analyses should be documented in a verifiable manner. In addition the basis for the choice of the analysed occurrences should be recorded. The results could for example be expressed like this: “The structure is robust against the influence”, “Risk reducing action ought to be carried out” or “The probability of the occurrences happening is too small—no action is required”. The result of the analysis should form a basis for decisions. Calculation of the structure’s ability to withstand the 10.000-year wave. Calculation of the structure’s stability during an earthquake combined with a hurricane and insufficient maintenance. Assessment of the consequences if all uncertainties in the dimensioning would be pulling in the same, unfortunate direction and at the same time 5% of the reinforcement would be lacking or seriously corroded in critical sections. Assessment of the consequences, if one of the structural elements should be torn off (as in the “Alexander L. Kielland”6 case).
6.
7. Examples: a) b) c)
d)
6
“Alexander L.Kielland” was a semi-submersible flotel which capsized in the Northern Sea on March 27, 1980. The direct/immediate cause of the loss was that one of the 5 platform legs was torn off, due to extremely heavy weather conditions.
Quality Audit/Quality System Audit
Quality Audit Systematic and independent examination to determine whether quality activities and related results comply with planned arrangements and whether these arrangements are implemented effectively and are suitable to achieve objectives. [ISO 8402:1994] Audit Program General view of the planned audits for a particular period of time. Audit Plan Detailed plan for the carrying out of a particular audit. Observation A statement of fact made during an audit and substantiated by objective evidence. [ISO 10011–1:1992] Objective Evidence Qualitative or quantitative information, records or statements of facts pertaining to the quality of an item or service or to the existence and implementation of a quality system element, which is based on observation, measurement or test and which can be verified. [ISO 10011– 1:1992], see also Appendix 3 Verification. Nonconformity Non-fulfilment of specified requirements. [ISO 8402:1994] Recommendation The audit team’s proposal for improvement of the auditee’s systems. Quality System Audit is carried out with one or more of the following intentions in mind: a) Ascertain whether the elements in the Quality System comply with the requirements of the company and the authorities, as a basis for pre-qualification of a supplier or contractor or as a part of the follow-up of a contract. b) Assess how effective the Quality System is implemented when it comes to meeting the goals for quality. c) Give the auditee an opportunity to improve the Quality System. 1. Quality Audits can and should contribute to a safe and efficient project execution, but it can never guarantee that nonconformity will not occur or that it is detected in time. The situation could be compared to the car-driving test (as basis for issuing a driving licence); the test is a means of ensuring safe driving, but it cannot guarantee that the driver will never cause accidents. The audit will in practice be based on spot tests, which, of course, affects the reliability.
The effectiveness of the audit depends on the competence of the auditor(s) and the auditee’s will and power to participate and cooperate, both during the audit and with regard to implementing corrective actions. • • • NPD: Regulations for structural design of loadbearing structures NPD: Regulations concerning the licensee’s internal control in petroleum activities ISO 9001, item 4.17
Reference requirements:
Principles:
1.
Regular quality audits are done in accordance with a drawn up audit program. Audits could also be caused by significant changes in the quality system or the quality of delivered products or services, or in order to follow up Corrective Action Requests (CAR). When selecting the areas to be audited, emphasis should be put on how critical the activity is when it comes to the safety and fitness for use of the product or service and the vulnerability regarding nonconformities. The Quality Audit typically applies to, but is not limited to, the quality system or elements thereof and to processes, products and services. Such audits may be called, respectively: * Quality System Audit * Process Quality Audit * Product Quality Audit * Service Quality Audit, etc. Quality System Audit is the one most frequently used. The basic principles are, however, the same for all of them. Quality Audit can also be termed according to when in the engineering and design or construction process it is carried out, e.g. in connection with pre-qualifying of supplier, or before commissioning. (The latter is often called Implementation Quality Audit or sometimes Implementation Review.) Audits should be carried out in accordance with ISO 10011–1, – 2 and –3; Guidelines for auditing quality systems. The standards give guidelines for the following conditions: a) The responsibilities and tasks of the audit team and audit leader. b) The qualification requirements to the auditors and the audit leader. c) Planning, preparation, carrying out and reporting the audit. d) Follow-up of Corrective Action Requests.
The Quality Audit should be reported and recorded in accordance with the guidelines in ISO 10011–1. The audit is not “closed” until it is verified and duly signed to show that all the Corrective Action Requests have been properly dealt with. * * ISO 9004–1, item 5.4 ISO 10011–1, –2, –3 Guidelines for auditing quality systems; Part 1: Auditing Part 2: Qualification criteria for quality system auditors Part 3: Management of audit programmes